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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 Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell ...

embryo
. Following
embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell ...
the fetal stage of development takes place. In human
prenatal development 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 ...

prenatal development
, fetal development begins from the ninth week after
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

fertilization
(or eleventh week
gestational age In obstetrics Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), ...
) and continues until
birth Birth is the act or process of bearing or bringing forth offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organ ...

birth
. 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
anatomical location
.


Etymology

The word ''
fetus A fetus American and British English spelling differences#ae and oe, 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 developm ...

fetus
'' (plural ''
fetuses
fetuses
'' or ''
feti
feti
'') is related to the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, 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 ...

Latin
''
fētus
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 ...

Ovid
in
Metamorphoses The ''Metamorphoses'' ( la, Metamorphōsēs, from grc, μεταμορφώσεις: "Transformations") is a Latin Narrative poetry, narrative poem from 8 Common Era, CE by the Ancient Rome, Roman poet Ovid. It is considered his ''Masterpiece, ...
, book 1, line 104. The predominant British, Irish, and Commonwealth spelling is ''
foetus
foetus
'', 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 the ...
, 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 member states of the Commonwealth of Nations, 56 member states, the vast majority of which are former territorial evolution of the British Empire ...

Commonwealth nations
, 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
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 , 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) 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 prenatal development, development. Muscles begin to move as soon as they ar ...
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 Cerebral cortex, cortices of the different senses (two cortices each, on left and right cerebral hemispheres, hemi ...
and
thalamus The thalamus (from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter located in the wikt:dorsal, dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the th ...

thalamus
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 scient ...

consciousness
,
dream A dream is a succession of images, ideas, emotions, and sensation (psychology), 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 ...

dream
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 (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

Iron
,
calcium Calcium is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemica ...

calcium
, and
phosphorus Phosphorus is a chemical element with the Symbol (chemistry), symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly Reactivity (chemistry), reactive, phosphor ...

phosphorus
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 infant, newborn. It is the first hair to be produced by the fetal hair follicles, and it usually appears around sixteen weeks of gestatio ...

lanugo
, 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 Human body, body matures into an adult body capable of sexual reproduction. It is initiated by hormone, hormonal signals from the brain to the gonads: the ovary, ovaries in a g ...

puberty
.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 (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

placenta
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 ...

weight
,
body mass index Body mass index (BMI) is a value derived from the mass (Mass versus weight, weight) and height of a person. The BMI is defined as the human body weight, body mass divided by the square (algebra), square of the human height, body height, and is ...

body mass index
, nutritional state, emotional stress, toxin exposure (including
tobacco Tobacco is the common name of several plants in the genus ''Nicotiana'' of the Family (biology), family Solanaceae, and the general term for any product prepared from the curing of tobacco, cured leaves of these plants. Nicotiana#Species, M ...

tobacco
,
alcohol Alcohol most commonly refers to: * Alcohol (chemistry) In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. The ...
,
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 brown ...

heroin
, and other drugs which can also harm the fetus in other ways), and
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 Cell (biology), cells, and transports Metabolic waste, metabolic waste products away from th ...

blood
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 Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...

hormone
output. Also, female fetuses tend to weigh less than males, at full term. Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (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 (obstetrics), gestational age. Infants born with LBW have added health risks which require close managemen ...
, although premature birth can also result in low birth weight. Low birth weight increases risk for perinatal mortality (
death Death is the Irreversible process, irreversible cessation of all biological process, biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whol ...

death
shortly after birth),
asphyxia Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the Human body, body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstance ...
,
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 ...
,
polycythemia Polycythemia (also known as polycythaemia) is a laboratory finding in which the hematocrit The hematocrit () (Ht or HCT), also known by #Names, several other names, is the volume fraction#Volume percent, volume percentage (vol%) of red blood ce ...
,
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, neurologic 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 babies are born, the main causes of 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 Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an el ...

heart
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 aw ...
s of the
circulatory system The blood circulatory system is a organ system, 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 vascula ...
, form relatively early during
embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell ...

embryonic development
, 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 chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
and nutrients from the mother through the
placenta The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

placenta
and the
umbilical cord In Placentalia, 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 i ...
.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 ...

umbilical vein
. About half of this enters the fetal '' ductus venosus'' and is carried to the
inferior vena cava The inferior vena cava is a large vein that carries the deoxygenated 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 Cell (biolog ...

inferior vena cava
, 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 ...

liver
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
heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an el ...

heart
. 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 Blood#Oxygen transport, deoxygenated blood returned from the body to the Right heart, right atrium of the heart where it is pumped out f ...
. 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 distribute ...

aorta
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 Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...

carbon dioxide
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 ...

pulmonary artery
. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the '' ductus arteriosus'', 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 The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent ...
). 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
gestational age In obstetrics Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), ...
) 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 The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and p ...

Pulmonary
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 ...

pulmonary arteries
from the
right atrium The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of k ...
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 The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of k ...
. The blood from the lungs travels through the
pulmonary vein The pulmonary veins are the veins that transfer Blood#Oxygen transport, 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 he ...
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 Cardiac muscle, muscular flap that is important in heart development. It is semilunar in shape, and grows downward from the upper wall of the Atrium (heart), atrium immediately to the right of the septum primum and ostium s ...
'', closing the ''foramen ovale'' and completing the separation of the newborn's
circulatory system The blood circulatory system is a organ system, 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 vascula ...
into the standard left and right sides. Thereafter, the ''foramen ovale'' is known as the ''fossa ovalis''. The '' ductus arteriosus'' 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
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 ...

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


Immune system

The
placenta The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

placenta
functions as a
maternal-fetal barrier The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

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 of infectious diseases can occur. Maternal IgG antibodies cross the placenta, giving the fetus
passive immunity Passive immunity is the transfer of active humoral immunity Humoral immunity is the aspect of immunity (medical), immunity that is mediated by macromolecules - including secreted antibodies, complement proteins, and certain antimicrobial peptides ...
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 ...
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 a ...
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 The spinal cord is a long, thin, tubular structure made up of nervous tissue, which ext ...
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 signific ...
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 fetal viability, able to survive independently. Miscarriage before 6 weeks of gestation is defined by ESHRE a ...
s and
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 (obstetrics), gestational age. Infants born with LBW have added health risks which require close managemen ...
(. 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 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...
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 childbirth, birth. A congenital heart defect is classed as a car ...
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 Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E ...
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 Birth is the act or process of bearing or bringing forth offspring In biology, offspring are the young creation of li ...
, 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 Ventricle (heart), ventricles of the heart. The extent of the opening may vary from pin size to complete absence of the ventricular sep ...
,
pulmonary atresia Pulmonary atresia is a congenital A birth defect, also known as a congenital disorder, is an abnormal condition that is present at childbirth, birth regardless of its cause. Birth defects may result in disability, disabilities that may be physi ...
, 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 Ectopic pregnancy is a Complications of pregnancy, complication of pregnancy in which the embryo attaches outside the uterus. Signs and symptoms classically include abdominal pain and ...
can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a lithopedion.


Fetal pain

Fetal pain, 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 (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occur ...
. 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 (psyc ...
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), am ...
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 Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A mu ...
s.


Other animals

A fetus is a stage in the
prenatal development 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 ...
of
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 organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). ...
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 List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (wh ...
.
Primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...
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 Marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant mars ...
varies from 18 days in jumping mice to 23 months in
elephant Elephants are the Largest and heaviest animals, 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 ...
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.


See also


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