Twins are two offspring produced by the same pregnancy. Twins can
be either monozygotic ("identical"), meaning that they develop from
one zygote, which splits and forms two embryos, or dizygotic
("fraternal"), meaning that they develop from two different eggs. In
fraternal twins, each twin is fertilized by its own sperm cell.
In contrast, a fetus that develops alone in the womb is called a
singleton, and the general term for one offspring of a multiple birth
is multiple. Non-related look-alikes whose resemblance parallels
that of twins are referred to as doppelgangers.
2 Types of twins and zygosity
2.1 Dizygotic (fraternal) twins
Monozygotic (identical) twins
2.2.3 Genetic and epigenetic similarity
Polar body and semi-identical twins
2.4 Unknown twin type
3 Degree of separation
4.2 Predisposing factors
5 Delivery interval
6 Complications during pregnancy
6.1 Vanishing twins
6.2 Conjoined twins
6.4 Parasitic twins
6.5 Partial molar twins
6.6 Miscarried twin
6.7 Low birth weight
6.8 Twin-to-twin transfusion syndrome
7 Management of birth
8 Human twin studies
8.1 Unusual twinnings
8.2 Semi-identical twins
8.3 Mirror image twins
9 Language development
11 See also
13 Further reading
The human twin birth rate in the
United States rose 76% from 1980
through 2009, from 18.9 to 33.3 per 1,000 births.
The Yoruba peoples have the highest rate of twinning in the world, at
45–50 twin sets (or 90–100 twins) per 1,000 live births,
possibly because of high consumption of a specific type of yam
containing a natural phytoestrogen which may stimulate the ovaries to
release an egg from each side.
In Central Africa, there are 18–30 twin sets (or 36–60 twins) per
1,000 live births. In Latin America, South Asia, and Southeast
Asia, the lowest rates are found; only 6 to 9 twin sets per 1,000 live
births. North America and Europe have intermediate rates of 9 to 16
twin sets per 1,000 live births.
Multiple pregnancies are much less likely to carry to full term than
single births, with twin pregnancies lasting on average 37 weeks,
three weeks less than full term.
Women who have a family history of fraternal twins have a higher
chance of producing fraternal twins themselves, as there is a
genetically linked tendency to hyper-ovulate. There is no known
genetic link for identical twinning. Other factors that increase
the odds of having fraternal twins include maternal age, fertility
drugs and other fertility treatments, nutrition, and prior births.
Types of twins and zygosity
The vast majority of twins are either dizygotic (fraternal) or
monozygotic (identical). Less common variants are discussed further
down the article.
Twins can be:
Female-female—sometimes called sororal twins
Male-male twins—no special name
Male–female twins—this is the most common pairing, simply by
virtue of it encompassing both "male-female" and "female-male" twins,
thus half of all fraternal (dizygotic) twins are male-female
Among non-twin births, male singletons are slightly (about five
percent) more common than female singletons. The rates for singletons
vary slightly by country. For example, the sex ratio of birth in the
US is 1.05 males/female, while it is 1.07 males/female in
Italy. However, males are also more susceptible than females to
die in utero, and since the death rate in utero is higher for twins,
it leads to female twins being more common than male twins.
Zygosity is the degree of identity in the genome of twins.
eggs are fertilized during different acts of intercourse
usage is practically equivalent with heteropaternal superfecundation,
which occurs when two different males father fraternal twins, because
though superfecundation by the same father is thought to be a common
occurrence, it can only be proven to have occurred with multiple
a woman gets pregnant again while already pregnant, resulting in
multiple fetuses at differing developmental stages
by definition only healthy fully formed fetus
fetal resorption, twin embolisation syndrome
up to 1 of every 8 multifetus pregnancies
by definition only healthy fully formed fetus
ranges from normal to compromised
Dizygotic (fraternal) twins
Eight-month-old sororal twins napping
Dizygotic (DZ) or fraternal twins (also referred to as "non-identical
twins", "dissimilar twins", "biovular twins", and, informally in the
case of females, "sororal twins") usually occur when two fertilized
eggs are implanted in the uterus wall at the same time. When two eggs
are independently fertilized by two different sperm cells, fraternal
twins result. The two eggs, or ova, form two zygotes, hence the terms
dizygotic and biovular. Fraternal twins are, essentially, two ordinary
siblings who happen to be born at the same time, since they arise from
two separate eggs fertilized by two separate sperm, just like ordinary
This is the most common type of twin.
Dizygotic twins, like any other siblings, have an extremely small
chance of having the same chromosome profile. Even if they happen to
have the same chromosome profile, they will always have different
genetic material on each chromosome, due to chromosomal crossover
during meiosis. Like any other siblings, dizygotic twins may look
similar, particularly given that they are the same age. However,
dizygotic twins may also look very different from each other.
Studies show that there is a genetic proclivity for dizygotic
twinning. However, it is only the mother who has any effect on the
chances of having such twins; there is no known mechanism for a father
to cause the release of more than one ovum. Dizygotic twinning ranges
from six per thousand births in Japan (similar to the rate of
monozygotic twins) to 14 and more per thousand in some African
Dizygotic twins are also more common for older mothers, with twinning
rates doubling in mothers over the age of 35. With the advent of
technologies and techniques to assist women in getting pregnant, the
rate of fraternals has increased markedly.
Monozygotic (identical) twins
Comparison of zygote development in monozygotic and dizygotic twins.
In the uterus, a majority of monozygotic twins (60–70%) share the
same placenta but have separate amniotic sacs. In 18–30% of
monozygotic twins each fetus has a separate placenta and a separate
amniotic sac. A small number (1–2%) of monozygotic twins share the
same placenta and amniotic sac. Fraternal twins each have their own
placenta and own amniotic sac.
Monozygotic (MZ) or identical twins occur when a single egg is
fertilized to form one zygote (hence, "monozygotic") which then
divides into two separate embryos.
Regarding spontaneous or natural monozygotic twinning, a recent theory
proposes that monozygotic twins are formed after a blastocyst
essentially collapses, splitting the progenitor cells (those that
contain the body's fundamental genetic material) in half, leaving the
same genetic material divided in two on opposite sides of the embryo.
Eventually, two separate fetuses develop. Spontaneous division of
the zygote into two embryos is not considered to be a hereditary
trait, but rather a spontaneous and random event.
Monozygotic twins may also be created artificially by embryo
splitting. It can be used as an expansion of in vitro fertilization
(IVF) to increase the number of available embryos for embryo
Monozygotic twinning occurs in birthing at a rate of about 3 in every
1000 deliveries worldwide. (about 0.6% of the world population).
The likelihood of a single fertilization resulting in monozygotic
twins is uniformly distributed in all populations around the
world. This is in marked contrast to dizygotic twinning, which
ranges from about six per thousand births in Japan (almost similar to
the rate of identical twins, which is around 4–5) to 15 and more per
thousand in some parts of India and up to over 20 in some Central
African countries. The exact cause for the splitting of a zygote
or embryo is unknown.
IVF techniques are more likely to create dizygotic twins. For IVF
deliveries, there are nearly 21 pairs of twins for every 1,000.
Genetic and epigenetic similarity
Monozygotic twins are genetically nearly identical and they are always
the same sex unless there has been a mutation during development. The
children of monozygotic twins test genetically as half-siblings (or
full siblings, if a pair of monozygotic twins reproduces with another
pair or with the same person), rather than first cousins. Identical
twins do not have the same fingerprints however, because even within
the confines of the womb, the fetuses touch different parts of their
environment, giving rise to small variations in their corresponding
prints and thus making them unique.
Monozygotic twins always have different phenotypes. Normally due to an
environmental factor or the deactivation of different X chromosomes in
female monozygotic twins, and in some extremely rare cases, due to
aneuploidy, twins may express different sexual phenotypes, normally
from an XXY
Klinefelter syndrome zygote splitting
Monozygotic twins, although genetically very similar, are not
genetically exactly the same. The DNA in white blood cells of 66 pairs
of monozygotic twins was analyzed for 506,786 single nucleotide
polymorphisms known to occur in human populations. Polymorphisms
appeared in 2 of the 33 million comparisons, leading the researchers
to extrapolate that the blood cells of monozygotic twins may have on
the order of one DNA-sequence difference for every 1.2 x 107
nucleotides, which would imply hundreds of differences across the
entire genome. The mutations producing the differences detected in
this study would have occurred during embryonic cell-division (after
the point of fertilization). If they occur early in fetal development,
they will be present in a very large proportion of body cells.
Another cause of difference between monozygotic twins is epigenetic
modification, caused by differing environmental influences throughout
Epigenetics refers to the level of activity of any
particular gene. A gene may become switched on, switched off, or could
become partially switched on or off in an individual. This epigenetic
modification is triggered by environmental events.
can have markedly different epigenetic profiles. A study of 80 pairs
of monozygotic twins ranging in age from three to 74 showed that the
youngest twins have relatively few epigenetic differences. The number
of epigenetic differences increases with age. Fifty-year-old twins had
over three times the epigenetic difference of three-year-old twins.
Twins who had spent their lives apart (such as those adopted by two
different sets of parents at birth) had the greatest difference.
However, certain characteristics become more alike as twins age, such
as IQ and personality.
Polar body and semi-identical twins
A 1981 study of a deceased triploid XXX twin fetus without a heart
showed that although its fetal development suggested that it was an
identical twin, as it shared a placenta with its healthy twin, tests
revealed that it was probably a polar body twin. The authors were
unable to predict whether a healthy fetus could result from a polar
In 2003, a study argued that many cases of triploidity arise from
Unknown twin type
In 2007, a study reported a case of a pair of living twins, one
intersex and one a phenotypical male. The twins were both found to be
chimeras and to share all of their maternal DNA but only half of their
father's DNA. The exact mechanism of fertilization could not be
determined but the study stated that it was unlikely to be a case of
polar body twinning.
Degree of separation
Various types of chorionicity and amniosity (how the baby's sac looks)
in monozygotic (one egg/identical) twins as a result of when the
fertilized egg divides
The degree of separation of the twins in utero depends on if and when
they split into two zygotes. Dizygotic twins were always two zygotes.
Monozygotic twins split into two zygotes at some time very early in
the pregnancy. The timing of this separation determines the
chorionicity (the number of placentae) and amniocity (the number of
sacs) of the pregnancy. Dichorionic twins either never divided (i.e.:
were dizygotic) or they divided within the first 4 days. Monoamnionic
twins divide after the first week.
In very rare cases, twins become conjoined twins. Non-conjoined
monozygotic twins form up to day 14 of embryonic development, but when
twinning occurs after 14 days, the twins will likely be conjoined.
Furthermore, there can be various degrees of shared environment of
twins in the womb, potentially leading to pregnancy complications.
It is a common misconception that two placentas means twins are
dizygotic. But if monozygotic twins separate early enough, the
arrangement of sacs and placentas in utero is indistinguishable from
Normally, twins have two separate (di- being a numerical prefix for
two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or
"DiDi". It occurs in almost all cases of dizygotic twins (except in
very rare cases of fusion between their blastocysts) and in
18–36% (or around 25%) of monozygotic (identical) twins.
DiDi twins have the lowest mortality risk at about 9 percent, although
that is still significantly higher than that of singletons.
Dichorionic-Diamniotic twins form when splitting takes place by the
third day after fertilization.
Monochorionic twins share the same placenta.
Monochorionic twins generally have two amniotic sacs (called
Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the
pregnancies with monozygotic twins, and in 0.3% of all
pregnancies. Monochorionic-Diamniotic twins are almost always
monozygotic, with a few exceptions where the blastocysts have
Monochorionic twins share the same placenta, and thus have a risk of
twin-to-twin transfusion syndrome.
Monochorionic twins share the same amnion in 1–2% of monozygotic
Monoamniotic twins are always monozygotic.
The survival rate for monoamniotic twins is somewhere between 50%
Monoamniotic twins, as with diamniotic monochorionic twins, have a
risk of twin-to-twin transfusion syndrome. Also, the two umbilical
cords have an increased chance of being tangled around the babies.
Because of this, there is an increased chance that the newborns may be
miscarried or suffer from cerebral palsy due to lack of oxygen.
Monoamniotic twins occur when the split takes place after the ninth
day after fertilization.
When the division of the developing zygote into 2 embryos occurs, 99%
of the time it is within 8 days of fertilization.
Mortality is highest for conjoined twins due to the many complications
resulting from shared organs.
If the division of the zygote occurs later than the 12 days then
conjoined twins are usually the result.
Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation
as part of IVF. The twin at left in the image is shown in the sagittal
plane with the head pointing towards upper left. The twin at right in
the image is shown in the coronal plane with the head pointing
Abdominal ultrasonography of monoamniotic twins at a gestational age
of 15 weeks. There is no sign of any membrane between the fetuses. A
coronal plane is shown of the twin at left, and a sagittal plane of
parts of the upper thorax and head is shown of the twin at right.
A 2006 study has found that insulin-like growth factor present in
dairy products may increase the chance of dizygotic twinning.
Specifically, the study found that vegan mothers (who exclude dairy
from their diets) are one-fifth as likely to have twins as vegetarian
or omnivore mothers, and concluded that "Genotypes favoring elevated
IGF and diets including dairy products, especially in areas where
growth hormone is given to cattle, appear to enhance the chances of
multiple pregnancies due to ovarian stimulation."
From 1980 to 1997, the number of twin births in the
United States rose
52%. This rise can at least partly be attributed to the increasing
popularity of fertility drugs and procedures such as IVF, which result
in multiple births more frequently than unassisted fertilizations do.
It may also be linked to the increase of growth hormones in food.
Main article: Populated places with highest incidence of multiple
A pair of female ere ibeji twin figures (early 20th-century) in the
permanent collection of The Children's Museum of Indianapolis. Yoruba
people have the highest twinning rate in the world.
About 1 in 90 human births (1.1%) results from a twin pregnancy.
The rate of dizygotic twinning varies greatly among ethnic groups,
ranging as high as about 45 per 1000 births (4.5%) for the Yoruba to
10% for Linha São Pedro, a tiny Brazilian settlement which belongs to
the city of Cândido Godói. In Cândido Godói, one in five
pregnancies has resulted in twins. The Argentine historian Jorge
Camarasa has put forward a theory that experiments of the
Josef Mengele could be responsible for the high ratio of twins in the
area. His theory was rejected by Brazilian scientists who had
studied twins living in Linha São Pedro; they suggested genetic
factors within that community as a more likely explanation. A high
twinning rate has also been observed in other places of the world,
Igbo-Ora in Nigeria
Kodinji, located in Kerala, India
Mohammadpur Umri, located in Uttar Pradesh, India
The widespread use of fertility drugs causing hyperovulation
(stimulated release of multiple eggs by the mother) has caused what
some call an "epidemic of multiple births". In 2001, for the first
time ever in the US, the twinning rate exceeded 3% of all births.
Nevertheless, the rate of monozygotic twins remains at about 1 in 333
across the globe.
In a study on the maternity records of 5750 Hausa women living in the
Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000
births. Twenty-six percent of twins were monozygotic. The incidence of
multiple births, which was about five times higher than that observed
in any western population, was significantly lower than that of other
ethnic groups, who live in the hot and humid climate of the southern
part of the country. The incidence of multiple births was related to
maternal age but did not bear any association to the climate or
prevalence of malaria.
The predisposing factors of monozygotic twinning are unknown.
Dizygotic twin pregnancies are slightly more likely when the following
factors are present in the woman:
She is of West African descent (especially Yoruba)
She is between the age of 30 and 40 years
She is greater than average height and weight
She has had several previous pregnancies.
Women undergoing certain fertility treatments may have a greater
chance of dizygotic multiple births. In the
United States it has been
estimated that by 2011 36% of twin births resulted from conception by
assisted reproductive technology.
The risk of twin birth can vary depending on what types of fertility
treatments are used. With in vitro fertilisation (IVF), this is
primarily due to the insertion of multiple embryos into the uterus.
Ovarian hyperstimulation without IVF has a very high risk of multiple
birth. Reversal of anovulation with clomifene (trade names including
Clomid) has a relatively less but yet significant risk of multiple
A 15-year German study of 8,220 vaginally delivered twins (that
is, 4,110 pregnancies) in
Hesse yielded a mean delivery time interval
of 13.5 minutes. The delivery interval between the twins was
measured as follows:
Within 15 minutes: 75.8%
16–30 minutes: 16.4%
31–45 minutes: 4.3%
46–60 minutes: 1.7%
Over 60 minutes: 1.8% (72 instances)
The study stated that the occurrence of complications "was found to be
more likely with increasing twin-to-twin delivery time interval" and
suggested that the interval be kept short, though it noted that the
study did not examine causes of complications and did not control for
factors such as the level of experience of the obstetrician, the wish
of the women giving birth, or the "management strategies" of the
procedure of delivering the second twin.
Complications during pregnancy
Main article: Vanishing twin
Researchers suspect that as many as 1 in 8 pregnancies start out as
multiples, but only a single fetus is brought to full term, because
the other fetus has died very early in the pregnancy and has not been
detected or recorded. Early obstetric ultrasonography exams
sometimes reveal an "extra" fetus, which fails to develop and instead
disintegrates and vanishes in the uterus. There are several reasons
for the "vanishing" fetus, including it being embodied or absorbed by
the other fetus, placenta or the mother. This is known as vanishing
twin syndrome. Also, in an unknown proportion of cases, two zygotes
may fuse soon after fertilization, resulting in a single chimeric
embryo, and, later, fetus.
Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the
origin of the term "Siamese twins".
Main article: Conjoined twins
Conjoined twins (or the once-commonly used term "siamese") are
monozygotic twins whose bodies are joined together during pregnancy.
This occurs when the zygote starts to split after day 12 following
fertilization and fails to separate completely. This condition occurs
in about 1 in 50,000 human pregnancies. Most conjoined twins are now
evaluated for surgery to attempt to separate them into separate
functional bodies. The degree of difficulty rises if a vital organ or
structure is shared between twins, such as the brain, heart or liver.
Main article: Chimera (genetics)
A chimera is an ordinary person or animal except that some of their
parts actually came from their twin or from the mother. A chimera may
arise either from monozygotic twin fetuses (where it would be
impossible to detect), or from dizygotic fetuses, which can be
identified by chromosomal comparisons from various parts of the body.
The number of cells derived from each fetus can vary from one part of
the body to another, and often leads to characteristic mosaicism skin
coloration in human chimeras. A chimera may be intersex, composed of
cells from a male twin and a female twin. In one case DNA tests
determined that a woman, mystifyingly, was not the mother of two of
her three children; she was found to be a chimera, and the two
children were conceived from eggs derived from cells of their mother's
Main article: Parasitic twin
Sometimes one twin fetus will fail to develop completely and continue
to cause problems for its surviving twin. One fetus acts as a parasite
towards the other. Sometimes the parasitic twin becomes an almost
indistinguishable part of the other, and sometimes this needs to be
Partial molar twins
A very rare type of parasitic twinning is one where a single viable
twin is endangered when the other zygote becomes cancerous, or molar.
This means that the molar zygote's cellular division continues
unchecked, resulting in a cancerous growth that overtakes the viable
fetus. Typically, this results when one twin has either triploidy or
complete paternal uniparental disomy, resulting in little or no fetus
and a cancerous, overgrown placenta, resembling a bunch of grapes.
Occasionally, a woman will suffer a miscarriage early in pregnancy,
yet the pregnancy will continue; one twin was miscarried but the other
was able to be carried to term. This occurrence is similar to the
vanishing twin syndrome, but typically occurs later, as the twin is
Low birth weight
It is very common for twins to be born at a low birth weight. More
than half of twins are born weighing less than 5.5 pounds
(2.5 kg), while the average birth weight of a healthy baby should
be around 6–8 pounds (3–4 kg). This is largely due to the
fact that twins are typically born premature. Premature birth and low
birth weights, especially when under 3.5 pounds (1.6 kg), can
increase the risk of several health-related issues, such as vision and
hearing loss, mental disabilities, and cerebral palsy. There is an
increased possibility of potential complications as the birth weight
of the baby decreases. Large mammals which give birth to twins is much
more rare. A recent birth of elephant twins in the Tarangire National
Park of Tanzania is a great example of how the birth of these two
healthy and thriving twins can beat the odds.
Twin-to-twin transfusion syndrome
Main article: Twin-to-twin transfusion syndrome
Monozygotic twins who share a placenta can develop twin-to-twin
transfusion syndrome. This condition means that blood from one twin is
being diverted into the other twin. One twin, the 'donor' twin, is
small and anemic, the other, the 'recipient' twin, is large and
polycythemic. The lives of both twins are endangered by this
Stillbirths occurs when a fetus dies after 20 weeks of gestation.
There are two types of stillbirth, including intrauterine death and
intrapartum death. Intrauterine death occurs when a baby dies during
late pregnancy. Intrapartum death, which is more common, occurs when a
baby dies while the mother is giving birth. The cause of stillbirth is
often unknown, but the rate of babies who are stillborn is higher in
twins and multiple births. Caesareans or inductions are advised after
38 weeks of pregnancy for twins, because the risk of stillbirth
increases after this time.
Management of birth
For otherwise healthy twin pregnancies where both twins are head down
a trial of vaginal delivery is recommended at between 37 and 38
Vaginal delivery in this case does not worsen the
outcome for the infant as compared with Caesarean section. There
is controversy on the best method of delivery where the first twin is
head first and the second is not. When the first twin is not head
down a caesarean section is often recommended. It is estimated
that 75% of twin pregnancies in the
United States were delivered by
caesarean section in 2008. In comparison, the rate of caesarean
section for all pregnancies in the general population varies between
14% and 40%. In twins that share the same placenta delivery may be
considered at 36 weeks.
Human twin studies
Twin studies are utilized in an attempt to determine how much of a
particular trait is attributable to either genetics or environmental
influence. These studies compare monozygotic and dizygotic twins for
medical, genetic, or psychological characteristics to try to isolate
genetic influence from epigenetic and environmental influence. Twins
that have been separated early in life and raised in separate
households are especially sought-after for these studies, which have
been used widely in the exploration of human nature. Classical twin
studies are now being supplemented with molecular genetic studies
which identify individual genes.
Among dizygotic twins, in rare cases, the eggs are fertilized at
different times with two or more acts of sexual intercourse, either
within one menstrual cycle (superfecundation) or, even more rarely,
later on in the pregnancy (superfetation). This can lead to the
possibility of a woman carrying fraternal twins with different fathers
(that is, half-siblings). This phenomenon is known as heteropaternal
superfecundation. One 1992 study estimates that the frequency of
heteropaternal superfecundation among dizygotic twins, whose parents
were involved in paternity suits, was approximately 2.4%; see the
references section, below, for more details.
Dizygotic twins from biracial couples can sometimes be mixed twins,
which exhibit differing ethnic and racial features. One such pairing
was born in
Germany in 2008 to a white father from
Germany and a black
mother from Ghana.
Heterotopic pregnancy is an exceedingly rare type of dizygotic
twinning in which one twin implants in the uterus as normal and the
other remains in the fallopian tube as an ectopic pregnancy. Ectopic
pregnancies must be resolved because they can be life-threatening to
the mother. However, in most cases, the intrauterine pregnancy can be
Among monozygotic twins, in extremely rare cases, twins have been born
with different sexes (one male, one female). When monozygotic
twins are born with different sexes it is because of chromosomal
defects. There can be monozygotic boy/girl twins if the sex gene of
the embryo has an extra x chromosome (the fertilized egg would be an
xxy) then when the egg splits, one can have xx (girl) genes and one
can have xy (boy) genes. This is rare, but possible. Records show
there are only 10 known cases of these type twins.
The probability of this is so small that multiples having different
sexes is universally accepted as a sound basis for in utero clinical
determination that the multiples are not monozygotic. Another
abnormality that can result in monozygotic twins of different genders
is if the egg is fertilized by a male sperm but during cell division
only the X chromosome is duplicated. This results in one normal male
(XY) and one female with
Turner syndrome (45,X). In these cases,
although the twins did form from the same fertilized egg, it is
incorrect to refer to them as genetically identical, since they have
Monozygotic twins can develop differently, due to different genes
being activated. More unusual are "semi-identical twins". These
"half-identical twins" are hypothesized to occur when an unfertilized
egg cleaves into two identical attached ova and which are viable for
fertilization. Both cloned ova are then fertilized by different sperm
and the coalesced eggs undergo further cell duplications developing as
a chimeric blastomere. If this blastomere then undergoes a twinning
event, two embryos will be formed, each of which have different
paternal genes and identical maternal genes.
This results in a set of twins with identical genes from the mother's
side, but different genes from the father's side. Cells in each fetus
carry genes from either sperm, resulting in chimeras. This form had
been speculated until only recently being recorded in western
Twin calves of the Hereford breed in Miles City, Montana
Mirror image twins
Mirror image twins result when a fertilized egg splits later in the
embryonic stage than normal timing, around day 9–12. This type of
twinning could exhibit characteristics with reversed asymmetry, such
as opposite dominant handedness, dental structure, or even organs
(situs inversus). If the split occurs later than this time period,
the twins risk being conjoined. There is no DNA-based zygosity test
that can determine if twins are indeed mirror image.
There have been many studies highlighting the development of language
in twins compared to single-born children. These studies have
converged on the notion that there is a greater rate of delay in
language development in twins compared to their single-born
counterparts. The reasons for this phenomenon are still in
question; however, cryptophasia was thought to be the major cause.
Idioglossia is defined as a private language that is usually invented
by young children, specifically twins. Another term to describe what
some people call "twin talk" is cryptophasia where a language is
developed by twins that only they can understand. The increased
focused communication between two twins may isolate them from the
social environment surrounding them.
Idioglossia has been found to be
a rare occurrence and the attention of scientists has shifted away
from this idea. However, there are researchers and scientists that say
cryptophasia or idioglossia is not a rare phenomenon. Current research
is looking into the impacts of a richer social environment for these
twins to stimulate their development of language.
Twins are common in many animal species, including cats, sheep,
ferrets, giant pandas, dogs, deer, marmosets, and tamarins. The
incidence of twinning among cattle is about 1–4%, and research is
under way to improve the odds of twinning, which can be more
profitable for the breeder if complications can be sidestepped or
managed. A female calf that is the twin of a bull becomes partially
masculinized and is known as a freemartin.
List of multiple births
List of twins
Incest between twins
Twins in mythology
Twin towns and sister cities
Wikimedia Commons has media related to:
Look up twin in Wiktionary, the free dictionary.
Wikiquote has quotations related to: Twins
^ MedicineNet > Definition of
Twin Last Editorial Review: 19 June
^ Michael R. Cummings "Human Heredity Principles and issues" p. 104.
^ "Twins, Triplets, Multiple Births: MedlinePlus". Nlm.nih.gov.
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Twin-to-twin transfusion syndrome
Twin reversed arterial perfusion
Parasitic twin (Craniopagus parasiticus,
Fetus in fetu)
BNF: cb11942304c (data)