A
twin is one of two
offspring produced in the same
pregnancy. Twins can either be ''identical'' (in scientific usage, "monozygotic"), meaning that they develop from one
zygote that splits and forms two embryos, or ''fraternal'' ("dizygotic") because they develop from two separate eggs that are fertilized by two separate
sperm.
In contrast, a fetus which develops alone in the womb is called a ''singleton'', and the general term for one offspring of a multiple birth is ''multiple''. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible. A less complex way to engineer genetically identical offspring is through the process of cloning, a procedure that has successfully been accomplished with several species of mammals.
Statistics
The twin birth rate in the United States is slightly above 32 twin live births per 1,000 live births, while the
Yoruba have the highest rate of twinning in the world, at 45 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.
Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.
Zygosity
Zygosity is the degree of identity in the
genome of twins.
There are five common variations of twinning. The three most common variations are all fraternal (dizygotic):
Male–female twins are the most common result, 50 percent of fraternal twins and the most common grouping of twins.
Female–female fraternal twins (sometimes called "sororal twins")
Male–male fraternal twins
The other two variations are identical (monozygotic) twins:
Female–female identical twins
Male–male identical twins (least common)
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 death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Fraternal/Sororal (dizygotic) twins
Fraternal or
dizygotic (
DZ)
twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases 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 and/or sororal twins, like any other siblings, have an extremely small chance of having the same chromosome profile. 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. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal and/or sororal twins are simply brothers and/or sisters who happen to be the same age.
Studies show that there is a genetic basis for dizygotic twinning. However, it is only their mother that has any effect on the chances of having fraternal and/or sororal 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 countries.
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.
===Identical (monozygotic) twins===
Identical or monozygotic (MZ) twins occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.
There are an estimated 11 million sets of identical twins and triplets in the world today.
Mechanism
Regarding spontaneous or ''natural'' monozygotic twinning, a recent theory posits that identical 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 or random event.
Identical twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.
Incidence
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.
The likelihood of a single fertilization resulting in identical twins is uniformly distributed in all populations around the world. This is in marked contrast to fraternal 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 24 in the US, which might mainly be due to IVF (in vitro fertilization). The exact cause for the splitting of a zygote or embryo is unknown.
In-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.
Genetic and epigenetic similarity
Identical twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. On rare occasions, identical twins may express different
phenotypes (normally due to an environmental factor or the deactivation of different X chromosomes in female identical twins), and in some extremely rare cases, due to
aneuploidy, twins may express different sexual
phenotypes, normally due to an XXY
Klinefelter's syndrome zygote splitting unevenly.
Identical twins actually have only nearly identical DNA, and differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between identical twins 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. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.
Phenotype similarity
Contrary to common opinion, identical twins are not always of the same phenotypical sex. There have been described cases where monozygocity resulted in 46,XO (i.e. female with
Turner syndrome) and 46,XY (i.e. male). This is thought to be due to unequal distribution of zygotic protoplasm. However, as a rule, traits and physical appearances of MZ twins are very similar.
Identical twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, identical twins often become less alike or more alike because of lifestyle choices or external influences. The children of identical twins would test genetically as half-siblings rather than first cousins.
Semi-identical twins
Half-identical or
semi-identical twins (also referred to as "half twins") are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. Although examples of half-identical twins have been found, the exact mechanism of their conception is not well-understood, but could theoretically occur in
polar body twinning where sperm cells fertilize both the
ovum and the second
polar body.
This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm. In this case, the ova are genetically identical.
Types
There are two mechanisms by which this might happen:
Polar twins (or "polar body twins"), where two sperm fertilize an
ovum, one of the two fertilizing a
polar body; or where an
ovum splits into identical copies, one containing a polar body, prior to
fertilization, allowing it to be fertilized by two different
sperm.
Sesquizygotic twins, where two
sperm fertilize the one ovum, forming a
triploid, and then splitting.
Incidence
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 likely a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. In 2003 a study argued that many cases of triploidity arise from semi-identical twinning. In 2007, a study reported a case of a pair of living twins, one a
hermaphrodite 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
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 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. 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 (non-identical). But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.
! Type
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! Description
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! Day
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Dichorionic-Diamniotic
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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 ), in 99.7% of all pregnancies, and in 18–36% (or around 25%) of monozygotic (identical) twins.
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DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.
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Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.
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Monochorionic twins share the same placenta.
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Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins. Monochorionic-Diamniotic twins are ''almost always'' monozygotic, with a few exceptions where the blastocysts have fused.
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Days 4-8
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When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization
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Sometimes, monochorionic twins also share the same amnion. This situation occurs in 1–2% of monozygotic twin pregnancies.
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Monoamniotic twins are ''always'' monozygotic (identical twins).
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The survival rate for monoamniotic twins is somewhere between 50% to 60%.
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Consequently, if twins are monoamniotic that means that the two babies will be sharing a placenta and as a result, due to the small capacity of sharing a sac, the umbilical cord has 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 the lack of oxygen.
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Monoamniotic twins occur when the split takes place after the ninth day after fertilization.
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Conjoined twins
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When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization.
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Mortality is highest for conjoined twins due to the many complications resulting from shared organs.
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If the division of the zygote occurs later than the 12 days then conjoined twins are usually the result.
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Demographics
A recent 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 1980–97, 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 like Clomid and procedures such as in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.
Ethnicity
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 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 have resulted in twins. The Argentine historian
Jorge Camarasa has put forward a theory that experiments of the
Nazi doctor
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, including
Igbo-Ora in
Nigeria and
Kodinji in
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 monozygous. 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.
Predisposing factors
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. This can vary depending on what types of fertility treatments are used. With in vitro fertilization (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, increasing the likelihood of multiples.
Delivery interval
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 of twin pregnancy
Vanishing twins
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 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. This is known as vanishing twin syndrome.
Conjoined twins
Conjoined twins (or the deprecated term "Siamese twins") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12 following fertilization. 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.
Chimerism
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.
Parasitic twins
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 medically dealt with.
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.
Miscarried twin
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 than the vanishing twin syndrome.
Low birth weight
Twins typically suffer from the lower
birth weights and greater likelihood of
prematurity that is more commonly associated with the higher multiple pregnancies.
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 condition.
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. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have are now being supplemented with molecular genetic studies which identify individual genes.
Unusual twinnings
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 salvaged.
Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that ''in utero'' multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.
Semi-identical twins
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 medicine.
Animal twins
Twins are common in many animal species, such as
cats,
sheep,
ferrets and
deer. 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. The nine-banded armadillo (''
Dasypus novemcinctus'') has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.
See also
Twins in mythology
Gemini (astrology)
Incest between twins
Conjoined twins
List of multiple births
Multiple birth
List of twins
Look-alike
Litter (animal)
References
Further reading
Bacon, Kate. ''Twins in Society: Parents, Bodies, Space, and Talk'' (Palgrave Macmillan; 2010) 221 pages; explores the experience of child twins, adult twins, and parents of twins, with a focus on Britain.
Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH.
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