A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

. Following

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

the fetal stage of development takes place. In human

prenatal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...

, fetal development begins from the ninth week after

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

(or eleventh week

gestational age In obstetrics, gestational age is a measure of the age of a pregnancy which is taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available. Suc ...

) and continues until

birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...

. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.

Etymology

The word ''

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...

'' (plural ''

fetuses A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...

'' or '' feti'') is related to the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

'' fētus'' ("offspring", "bringing forth", "hatching of young") and the Greek "φυτώ" to plant. The word "fetus" was used by

Ovid Pūblius Ovidius Nāsō (; 20 March 43 BC – 17/18 AD), known in English as Ovid ( ), was a Roman poet who lived during the reign of Augustus. He was a contemporary of the older Virgil and Horace, with whom he is often ranked as one of the th ...

Metamorphoses The ''Metamorphoses'' ( la, Metamorphōsēs, from grc, μεταμορφώσεις: "Transformations") is a Latin narrative poem from 8 CE by the Roman poet Ovid. It is considered his ''magnum opus''. The poem chronicles the history of the wo ...

, book 1, line 104. The predominant British, Irish, and Commonwealth spelling is ''

foetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...

'', which has been in use since at least 1594. The spelling with ''-oe-'' arose in

Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the , and continuing into the 7th century in t ...

, in which the distinction between the vowel sounds ''-oe-'' and ''-e-'' had been lost. This spelling is the most common in most

Commonwealth nations The Commonwealth of Nations, simply referred to as the Commonwealth, is a political association of 56 member states, the vast majority of which are former territories of the British Empire. The chief institutions of the organisation are the Co ...

, except in the medical literature, where ''fetus'' is used. The more classical spelling ''fetus'' is used in Canada and the United States. In addition, ''fetus'' is now the standard English spelling throughout the world in medical journals. The spelling '' faetus'' was also used historically.

Development in humans

Weeks 9 to 16 (2 to 3.6 months)

In humans, the fetal stage starts nine weeks after fertilization.Klossner, N. Jayne
Introductory Maternity Nursing
(2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth" At the start of the fetal stage, the fetus is typically about in length from crown-rump, and weighs about 8 grams. The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.
The Columbia Encyclopedia
'' (Sixth Edition). Retrieved 2007-03-05. At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.Prechtl, Heinz
"Prenatal and Early Postnatal Development of Human Motor Behavior"
in ''Handbook of brain and behaviour in human development'', Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers): "The first movements to occur are sideward bendings of the head. ... At 9-10 weeks postmestrual age complex and generalized movements occur. These are the so-called general movements (Prechtl et al., 1979) and the startles. Both include the whole body, but the general movements are slower and have a complex sequence of involved body parts, while the startle is a quick, phasic movement of all limbs and trunk and neck."

Weeks 17 to 25 (3.6 to 6.6 months)

A woman pregnant for the first time (

nulliparous In biology and human medicine, gravidity and parity are the number of times a woman is or has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These terms are usually coupled, sometimes with additional te ...

) typically feels

fetal movement Fetal movement refers to motion of a fetus caused by its own muscle activity. Locomotor activity begins during the late embryological stage and changes in nature throughout development. Muscles begin to move as soon as they are innervated. These fi ...

s at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks. By the end of the fifth month, the fetus is about long.

Weeks 26 to 38 (6.6 to 8.6 months)

The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the

sensory cortex The sensory cortex can refer informally to the primary somatosensory cortex, or it can be used as a term for the primary and secondary cortices of the different senses (two cortices each, on left and right hemisphere): the visual cortex on the occi ...

and

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

develop as early as 24 weeks' gestational age, but the first evidence of their function does not occur until around 30 weeks, when minimal

consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...

dream A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, althou ...

ing, and the ability to feel pain emerges. Bones are fully developed, but are still soft and pliable.

Iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

, and

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

become more abundant. Fingernails reach the end of the fingertips. The

lanugo Lanugo is very thin, soft, usually unpigmented, downy hair that is sometimes found on the body of a fetus or newborn. It is the first hair to be produced by the fetal hair follicles, and it usually appears around sixteen weeks of gestation and is ...

, or fine hair, begins to disappear, until it is gone except on the upper arms and shoulders. Small breast buds are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week after fertilization. The fetus is considered full-term between weeks 37 and 40, when it is sufficiently developed for life outside the uterus. It may be in length, when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. ...

.Becher, Julie-Claire. , ''Behind the Medical Headlines'' (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)

Variation in growth

There is much variation in the growth of the human fetus. When fetal size is less than expected, the condition is known as

intrauterine growth restriction Intrauterine growth restriction (IUGR), or fetal growth restriction, refers to poor growth of a fetus while in the womb during pregnancy. IUGR is defined by clinical features of malnutrition and evidence of reduced growth regardless of an infant's ...

also called fetal growth restriction; factors affecting fetal growth can be ''maternal'', ''

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...

l'', or ''fetal''.Holden, Chris and MacDonald, Anita.
Nutrition and Child Health
' (Elsevier 2000). Retrieved 2007-03-04. Maternal factors include maternal

weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a Euclidean vector, vector quantity, the gravitational force acting on the object. Others define weigh ...

body mass index Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms and he ...

, nutritional state,

emotional stress In psychology, stress is a feeling of emotional strain and pressure. Stress is a type of psychological pain. Small amounts of stress may be beneficial, as it can improve athletic performance, motivation and reaction to the environment. Excess ...

, toxin exposure (including

tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

heroin Heroin, also known as diacetylmorphine and diamorphine among other names, is a potent opioid mainly used as a recreational drug for its euphoric effects. Medical grade diamorphine is used as a pure hydrochloride salt. Various white and brow ...

, and other drugs which can also harm the fetus in other ways), and

uterine The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the hollow organ, organ in the female reproductive system, reproductive system of most female mammals, including humans that accommodates the embryonic development, embryonic an ...

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...

flow. Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization and nutrient production. Fetal factors include the fetal genome, nutrient production, and

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

output. Also, female fetuses tend to weigh less than males, at full term. Fetal growth is often classified as follows:

small for gestational age Small for gestational age (SGA) newborns are those who are smaller in size than normal for the gestational age, most commonly defined as a weight below the 10th percentile for the gestational age. Causes Being small for gestational age is broadly ...

(SGA),

appropriate for gestational age Prenatal development () includes the embryonic development, development of the embryo and of the fetus during a viviparity, viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic developm ...

(AGA), and

large for gestational age Large for gestational age (LGA) is a term used to describe infants that are born with an abnormally high weight, specifically in the 90th percentile or above, compared to other babies of the same developmental age. Macrosomia is a similar term tha ...

(LGA). SGA can result in

low birth weight Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal intensi ...

, although premature birth can also result in low birth weight. Low birth weight increases risk for perinatal mortality (

death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

shortly after birth),

asphyxia Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...

hypothermia Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe h ...

polycythemia Polycythemia (also known as polycythaemia) is a laboratory finding in which the hematocrit (the volume percentage of red blood cells in the blood) and/or hemoglobin concentration are increased in the blood. Polycythemia is sometimes called erythr ...

hypocalcemia Hypocalcemia is a medical condition characterized by low calcium levels in the blood serum. The normal range of blood calcium is typically between 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L) while levels less than 2.1 mmol ...

immune dysfunction An immune disorder is a dysfunction of the immune system. These disorders can be characterized in several different ways: * By the component(s) of the immune system affected * By whether the immune system is overactive or underactive * By whether t ...

neurologic Neurology (from el, νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the brain, the spinal c ...

abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.

Viability

Fetal viability Fetal viability is the ability of a human fetus to survive outside the uterus. Medical viability is generally considered to be between 23 and 24 weeks Gestational age (obstetrics), gestational age. Viability depends upon factors such as birth weigh ...

refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately months gestational age and is usually later. There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.Moore, Keith and Persaud, T
''The Developing Human: Clinically Oriented Embryology''
p. 103 (Saunders 2003). According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation ( months); 50–70% at 24–25 weeks (6 – months); and >90% at 26–27 weeks ( – months) and over.March of Dimes - Neonatal Death
, retrieved September 2, 2009. It is rare for a baby weighing less than to survive. When such

premature Premature may refer to: * ''Premature'' (2014 film), an American comedy film * ''Premature'' (2019 film), an American romantic drama film * ''PREMature'', a 2015 British television drama miniseries See also * Premature aging, of an organism * P ...

babies are born, the main causes of

mortality Mortality is the state of being mortal, or susceptible to death; the opposite of immortality. Mortality may also refer to: * Fish mortality, a parameter used in fisheries population dynamics to account for the loss of fish in a fish stock throug ...

are that the respiratory system and the central nervous system are not completely differentiated. If given expert postnatal care, some preterm babies weighing less than may survive, and are referred to as ''extremely low birth weight'' or ''immature infants''.

Preterm birth Preterm birth, also known as premature birth, is the Childbirth, birth of a baby at fewer than 37 weeks Gestational age (obstetrics), gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 we ...

is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths. At an occurrence rate of 5% to 18% of all deliveries, it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.

Circulatory system

Before birth

The

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

and

blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...

s of the

circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, form relatively early during

, but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity, since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

and nutrients from the mother through the

and the

umbilical cord In placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologic ...

.Whitaker, Kent.
Comprehensive Perinatal and Pediatric Respiratory Care
' (Delmar 2001). Retrieved 2007-03-04. Blood from the placenta is carried to the fetus by the

umbilical vein The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood vo ...

. About half of this enters the fetal ''

ductus venosus In the fetus, the ''ductus venosus'' (Arantius' duct after Julius Caesar Aranzi) shunts a portion of umbilical vein blood flow directly to the inferior vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver. Compared to ...

'' and is carried to the

inferior vena cava The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of the ...

, while the other half enters the

liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...

proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the

portal vein The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approxima ...

. The blood then moves to the right atrium of the

. In the fetus, there is an opening between the right and left atrium (the '' foramen ovale''), and most of the blood flows from the right into the left atrium, thus bypassing

pulmonary circulation The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. ...

. The majority of blood flow is into the left ventricle from where it is pumped through the

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...

into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

and other waste products from the fetus are taken up and enter the mother's circulation. Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the

pulmonary artery A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and t ...

. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ''

ductus arteriosus The ''ductus arteriosus'', also called the ''ductus Botalli'', named after the Italian physiologist Leonardo Botallo, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta. It allo ...

'', which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

). File:Ultrasound_image_of_a_fetus.jpg,

3D ultrasound 3D ultrasound is a medical ultrasound technique, often used in fetal, cardiac, trans-rectal and intra-vascular applications. 3D ultrasound refers specifically to the volume rendering of ultrasound data. When involving a series of 3D volumes collec ...

of fetus (about months

) File:Sucking his thumb and waving.jpg, Fetus at months File:3dultrasound 20 weeks.jpg, Fetus at 5 months

Postnatal development

With the first breath after birth, the system changes suddenly.

Pulmonary The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...

resistance is reduced dramatically, prompting more blood to move into the

pulmonary arteries A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and t ...

from the

right atrium The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves. There are two atr ...

and ventricle of the heart and less to flow through the '' foramen ovale'' into the

left atrium The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves. There are two atr ...

. The blood from the lungs travels through the

pulmonary vein The pulmonary veins are the veins that transfer oxygenated blood from the lungs to the heart. The largest pulmonary veins are the four ''main pulmonary veins'', two from each lung that drain into the left atrium of the heart. The pulmonary vein ...

s to the left atrium, producing an increase in pressure that pushes the ''

septum primum During heart development of a human embryo, the single primitive atrium becomes divided into right and left by a , the septum primum. The septum primum () grows downward into the single atrium. Development The gap below it is known as the ostium ...

'' against the ''

septum secundum The septum secundum is a muscular flap that is important in heart development. It is semilunar in shape, and grows downward from the upper wall of the atrium immediately to the right of the septum primum and ostium secundum. It is important in the ...

'', closing the ''foramen ovale'' and completing the separation of the newborn's

into the standard left and right sides. Thereafter, the ''foramen ovale'' is known as the ''fossa ovalis''. The ''

'' normally closes within one or two days of birth, leaving the ''

ligamentum arteriosum The ligamentum arteriosum (arterial ligament), also known as the Ligament of Botallo or Harvey's ligament, is a small ligament attaching the aorta to the pulmonary artery. It serves no function in adults but is the remnant of the ductus arteriosus ...

'', while the

and ''

'' usually closes within two to five days after birth, leaving, respectively, the liver's '' ligamentum teres'' and '' ligamentum venosus''.

Immune system

The

functions as a maternal-fetal barrier against the transmission of

microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s. When this is insufficient,

mother-to-child transmission A vertically transmitted infection is an infection caused by pathogenic bacteria or viruses that use mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It can ...

of infectious diseases can occur. Maternal

IgG antibodies Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG ...

cross the placenta, giving the fetus

passive immunity Passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially, when hi ...

against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.

Developmental problems

A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of

birth defect A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

s. One area of concern is the lifestyle choices made during pregnancy. Diet is especially important in the early stages of development. Studies show that supplementation of the person's diet with

folic acid Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...

reduces the risk of

spina bifida Spina bifida (Latin for 'split spine'; SB) is a birth defect in which there is incomplete closing of the spine and the membranes around the spinal cord during early development in pregnancy. There are three main types: spina bifida occulta, me ...

and other

neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, a ...

defects. Another dietary concern is whether breakfast is eaten. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of prematurity, or birth defects. Alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to

intellectual disability Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation,Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signifi ...

in some infants. Smoking during pregnancy may also lead to

miscarriage Miscarriage, also known in medical terms as a spontaneous abortion and pregnancy loss, is the death of an embryo or fetus before it is able to survive independently. Miscarriage before 6 weeks of gestation is defined by ESHRE as biochemical lo ...

s and

(. Low birth weight is a concern for medical providers due to the tendency of these infants, described as "''premature'' by weight", to have a higher risk of secondary medical problems.

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits. Congenital disorders are acquired before birth. Infants with certain

congenital heart defect A congenital heart defect (CHD), also known as a congenital heart anomaly and congenital heart disease, is a defect in the structure of the heart or great vessels that is present at birth. A congenital heart defect is classed as a cardiovascular ...

s can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of

prostaglandin The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derive ...

s to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of

patent ductus arteriosus ''Patent ductus arteriosus'' (PDA) is a medical condition in which the ''ductus arteriosus'' fails to close after birth: this allows a portion of oxygenated blood from the left heart to flow back to the lungs by flowing from the aorta, which has ...

, where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided. Other heart birth defects include

ventricular septal defect A ventricular septal defect (VSD) is a defect in the ventricular septum, the wall dividing the left and right ventricles of the heart. The extent of the opening may vary from pin size to complete absence of the ventricular septum, creating one ...

pulmonary atresia Pulmonary atresia is a congenital malformation of the pulmonary valve in which the valve orifice fails to develop. The valve is completely closed thereby obstructing the outflow of blood from the heart to the lungs. The pulmonary valve is located o ...

, and

tetralogy of Fallot Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are: *pulmonary stenosis, which is narrowing of the exit from the ri ...

. An

abdominal pregnancy An abdominal pregnancy is a rare type of ectopic pregnancy where the embryo or fetus is growing and developing outside the uterus, womb in the abdomen, but not in the Fallopian tube (usual location), ovary or broad ligament. Because tubal, ovari ...

can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a

lithopedion A lithopedion ( also spelled lithopaedion or lithopædion; from grc, λίθος "stone" and grc, παιδίον "small child, infant"), or stone baby, is a rare phenomenon which occurs most commonly when a fetus dies during an abdominal pregna ...

Fetal pain

Fetal pain Prenatal perception is the study of the extent of somatosensory and other types of perception during pregnancy. In practical terms, this means the study of fetuses; none of the accepted indicators of perception are present in embryos. Studies in th ...

, its existence and its implications are debated politically and academically. According to the conclusions of a review published in 2005, "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester." Two authors of the study published in JAMA did not report their abortion-related activities, which pro-life groups called a conflict of interest; the editor of JAMA responded that JAMA probably would have mentioned those activities if they had been disclosed, but still would have published the study. See Denise Grady
"Study Authors Didn't Report Abortion Ties"
, ''New York Times'' (2005-08-26)."Study: Fetus feels no pain until third trimester"
NBC News However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about months) is an essential event with regard to fetal perception of pain.Johnson, Martin and Everitt, Barry.
Essential reproduction
' (Blackwell 2000): "The multidimensionality of pain perception, involving sensory, emotional, and cognitive factors may in itself be the basis of conscious, painful experience, but it will remain difficult to attribute this to a fetus at any particular developmental age." Retrieved 2007-02-21. Nevertheless, the perception of pain involves sensory, emotional and cognitive factors and it is "impossible to know" when pain is experienced, even if it is known when thalamocortical connections are established. Some authors argue that fetal pain is possible from the second half of

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...

. Evidence suggests that the perception of pain in the fetus occurs well before late gestation Glover V. The fetus may feel pain from 20 weeks. ''Conscience''. 2004-2005 Winter;25(3):35-7 Whether a fetus has the ability to feel pain and

suffering Suffering, or pain in a broad sense, may be an experience of unpleasantness or aversion, possibly associated with the perception of harm or threat of harm in an individual. Suffering is the basic element that makes up the negative valence of a ...

is part of the

abortion debate The abortion debate is a longstanding, ongoing controversy that touches on the moral, legal, medical, and religious aspects of induced abortion. In English-speaking countries, the debate most visibly polarizes around adherents of the self-describ ...

. In the United States, for example, anti-abortion advocates have proposed legislation that would require providers of abortions to inform pregnant women that their fetuses may feel pain during the procedure and that would require each person to accept or decline

anesthesia Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), ...

for the fetus.

Legal and social issues

Abortion of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit

late-term abortion Late termination of pregnancy, also referred to as late-term abortion, describes the termination of pregnancy by induced abortion during a late stage of gestation. In this context, ''late'' is not precisely defined, and different medical publicati ...

Other animals

A fetus is a stage in the

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

organisms. This stage lies between

embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

and birth. Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of

onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

and many

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s. The fetuses of most mammals are situated similarly to the human fetus within their mothers. However, the anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female. Development at birth varies considerably among animals, and even among mammals.

Altricial In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...

species are relatively helpless at birth and require considerable parental care and protection. In contrast,

precocial In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...

animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against,

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

Primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

s are precocial at birth, with the exception of humans.Lewin, Roger
Human Evolution
page 78 (Blackwell 2004). The duration of gestation in

placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

varies from 18 days in

jumping mice Zapodidae, the jumping mice, is a family of mouse-like rodents in North America and China. Although mouse-like in general appearance, these rodents are distinguished by their elongated hind limbs, and, typically, by the presence of four pairs o ...

to 23 months in

elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...

s.Sumich, James and Dudley, Gordon
Laboratory and Field Investigations in Marine Life
page 320 (Jones & Bartlett 2008). Generally speaking, fetuses of larger land mammals require longer gestation periods. The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage). In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.

References

External links

Prenatal Image Gallery Index
at the Endowment for Human Development website, featuring numerous motion pictures of human fetal movement.
''In the Womb''
(National Geographic video).

MedlinePlus Medical Encyclopedia {{Authority control Animal developmental biology Embryology Fertility