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File:TwinGirls.jpg
Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic (MZ, colloquially "identical") or dizygotic (DZ, colloquially "fraternal" or "non-identical").

==Definitions==The general term for more than one offspring in the same pregnancy (multiple birth) is multiples; a fetus which develops alone in the womb is called a singleton.

==Statistics==
The number of living human twins in the world has been estimated to be approximately 125 million in 2006<ref>{{cite web
|url=http://www.esrcsocietytoday.ac.uk/ESRCInfoCentre/about/CI/CP/Our_Society_Today/News_Articles_2006/twins.aspx
|title=Twin Resources
|publisher=Economic and Social Research Council
|year=2006
|last=Oliver
|first=Judith|accessdate=2009-04-21}}</ref> (roughly 1.9% of the world population), with just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The twin birth rate in the United States in 2004,2005 and 2006 was slightly above 32 twin live births per 1,000 live births<ref>Martin, Joyce A.; Hamilton, Brady E.; Sutton, Paul D.; Ventura, Stephanie J.; Menacker, Fay; Kirmeyer, Sharon and Mathews, T.J. Births: Final Data for 2006, National Vital Statistics Report, Vol. 57, No. 7, Jan. 7, 2009, 102 pp. (Table 39, pp.83-84).</ref>.

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.<ref>{{cite journal
|journal=Clinical Perinatology
|year=2007
|month=December
|issue=4
|pages=599–609
|last=Elliott
|first=JP
|title=Preterm labor in twins and high-order multiples.
|quote=Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.
|pmid=18063108
|volume=34|doi=10.1016/j.clp.2007.10.004}}</ref> Since premature births can have health consequences for the babies, twin births are often handled with special precautions.

The Yoruba, a large Nigerian ethnic group, have the highest rate of twinning in the world, at 45 twins per 1,000 live births.<ref>{{cite web
|url=http://www.emedicine.com/PED/topic2599.htm
|first=Terence
|last=Zach
|coauthors=Arun K Pramanik and Susannah P Ford
|title=Multiple Births
|date=2007-10-02
|publisher=WebMD
|accessdate=2008-09-29}}</ref><ref>{{cite news
|url=http://www.iol.co.za/index.php?from=rss_Africa&set_id=1&click_id=68&art_id=nw20071112091221822C182549
|title=Genetics or yams in the Land of Twins?
|publisher=Independent Online
|date=2007-11-12
|accessdate=2008-09-29}}</ref><ref>{{cite news
|url=http://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml
|title=The Land Of Twins
|publisher=BBC World Service
|date=2001-06-07
|accessdate=2008-09-29}}</ref> Some researchers have claimed this may be because of high consumption of a specific type of yam, Dioscorea rotundata or white yam containing a natural hormone phytoestrogen which may stimulate the ovaries to release an egg from each side.<ref name="Keith">{{cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith, Emile Papierik, Donald M. Keith and Barbara Luke
|author=O. Bomsel-Helmreich
|coauthors=W. Al Mufti
|isbn=1-85070-666-2
|publisher=Taylor and Francis
|chapter=The mechanism of monozygosity and double ovulation
|year=1995
|pages=34}}</ref><ref>{{cite web
|url=http://yalemedicine.yale.edu/ym_su99/scope/scope1.htm#yam
|title=What’s in a yam? Clues to fertility, a student discovers
|year=1999|accessdate=2008-09-29}}</ref>

==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 dizygotic:
*Male–female twins are the most common result, 50 percent of DZ twins and the most common grouping of twins.
*Female DZ twins (sometimes called sororal twins)
*Male DZ twins
The other two variations are monozygotic twins:
*Female MZ twins*Male MZ 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,<ref>Template:Cite web</ref> while it is 1.07 males/female in Italy.<ref>Template:Cite web</ref> 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.

File:Being a twin means you always have a pillow or blanket handy.jpg

===Dizygotic twins===
Dizygotic twins (commonly known as fraternal twins, but also referred to as non-identical twins or biovular twins) usually occur when two fertilized eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different sperm cells, DZ twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.
 Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, DZ twins may look similar, particularly given that they are the same age. However, DZ 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 DZ twins are simply brothers and/or sisters who happen to be the same age.

Studies show that there is a genetic basis for DZ twinning. However, it is only their mother that has any effect on the chances of having DZ 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.<ref name="Keith">{{cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith
|author=R. Derom
|coauthors=J. Orlebeke, A. Eriksson, M. Thiery
|isbn=1850706662
|publisher=Taylor and Francis
|chapter=The epidemiology of multiple births in Europe
|year=1995
|pages=145}}</ref>

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.<ref name="Bortolus">{{cite journal
|title=The epidemiology of multiple births
|journal=Human Reproduction Update
|year=1999
|volume=5
|issue=2
|pages=179–187
|last=Bortolus
|first=Renata
|coauthors=Fabio Parazzini, Liliane Chatenoud, Guido Benzi, Massimiliano Maria Bianchi, Alberto Marini
|publisher=European Society of Human Reproduction and Embryology
|issn=1355-4786
|pmid=10336022
|doi=10.1093/humupd/5.2.179}}</ref> With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

===Monozygotic twins===<!-- This section is linked from Rheumatoid arthritis -->
File:Identical-fraternal-sperm-egg.png
Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos.

Monozygotic twins are almost always the same sex and their traits and physical appearances are very similar but not exactly the same; although they have nearly identical DNA,<ref>Template:Cite news</ref> environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,<ref>Template:Cite web</ref> regardless of race. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result.

Monozygotic twins are genetically identical (unless there has been a mutation during development) and they are almost always the same sex.  On rare occasions, monozygotic twins may express different phenotypes, normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female 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<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>). Monozygotic twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. It is estimated that there are around 10 million monozygotic twins and triplets in the world.

The likelihood of a single fertilisation resulting in MZ twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.<ref name="Bortolus" /> This is in marked contrast to DZ twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4–5) to 15 and more per thousand in some parts of India<ref>Template:Cite journal</ref> and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.

Monozygotic twins have nearly identical DNA, but 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 MZ 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.<ref>Template:Cite journal</ref> However, certain characteristics become more alike as twins age, such as IQ and personality.<ref>Template:Cite bookTemplate:Pn</ref><ref>Template:Cite bookTemplate:Pn</ref> This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.

A recent theory posits 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. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting of the European Society of Human Reproduction and Embryology in Lyon, France. Utilizing computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, documented for the first time the early days of twin development. Payne also discovered explanation for why 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.<ref>Template:Cite news</ref><ref>Template:Cite press release</ref>

===Half twin===

Main article: Half twinThe extremely rare half twins or semi-identical twins' are twins that inherit exactly the same genes from their mother but different genes from their father. The exact mechanism of their conception is not well-understood, but could theoretically occur in in polar body twinning where sperm cells fertilize both the ovum and the second polar body.
==Degree of separation==
File:Placentation.svg

In very rare cases, twins become conjoined twins. Furthermore, there is a various degree of shared environment of twins in the womb, potentially leading to pregnancy complications.

===Dichorionic-Diamniotic===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<ref name=Shulman/> ), in 99.7% of all pregnancies,<ref name=cordero>Template:Cite journal</ref> and in 18–36%<ref name="splitstats">Template:Cite web</ref> (or around 25%<ref name=Shulman>Template:Cite book</ref>) of monozygotic (identical) twins. Dichorionic-Diamniotic twins form when splitting takes place after the third day after fertilization.<ref name=Shulman/>

DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. <ref>Template:Cite book</ref>

===Monochorionic===

Main article: Monochorionic twinsMonochorionic 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.<ref name="splitstats" /> Sometimes, however, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization. <ref name=Shulman/> Monoamniotic twins are always monozygotic (identical twins).<ref name=pregnancy-info> Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009</ref> Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. <ref name=Shulman/>

MoDi twins have about a 25 percent mortality due to twin-to-twin transfusion syndrome.Template:Fact

===Monochorionic-Monoamniotic===

Main article: Monoamniotic twinsMonoamniotic twins also share the same amniotic sac. Monoamniotic twins are always monozygotic.<ref name=pregnancy-info> Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009</ref> Monochorionic twins, on the other hand, may be dizygotic. Furthermore, two placentas (dichorionic twins) does not necessarily exclude monozygocity since monozygotic twins can have two placentas. Chorionicity and amniocity are a result of the division time. Dichorionic twins divide within the first 4 days. Monoamnionic twins divide after the first week.

It occurs in 1–2% of monozygotic twin pregnancies.<ref name="splitstats" />

The survival rate for monoamniotic twins is somewhere between 50%<ref name=pregnancy-info/> to 60%<ref name=about> MoMo Twins; Monochorionic Monoamniotic Twins By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009</ref>.

Consequently, if twins are monoamniotic that means that the two babies will be sharing an umbilical cord and as a result, due to the small capacity of sharing a sac, the umbilical cord has an increase chance of being tangled around the babies. Because of this, there is an increase chance that the newborns may be a miscarriage due to the lack of oxygen, or maybe be born mentally challenged due to the lack of oxygen reaching the brain.

===Conjoined===

Main article: Conjoined twinsWhen the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. If the division of the zygote occurs later than the 8 days then conjoined twins are usually the result.

Mortality is highest for conjoined twins due to the many complications resulting from shared organs.

==Demographics==A recent study found that vegan mothers are five times less likely to have twins than 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."<ref name="vegan">Template:Cite journal</ref>

From 1980–97, the number of twin births in the United States rose 52%.<ref>Template:Cite journal</ref> 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.<ref name="vegan" />

===Ethnicity===
About 1 in 90 human births (1.1%) results from a twin pregnancy.<ref name="asch">Template:Cite bookTemplate:Pn</ref> 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.<ref>Template:Cite journal</ref> In Cândido Godói, one in five pregnancies have resulted in twins – most of them blond haired and blue eyed.<ref>{{cite news
 | url = http://www.telegraph.co.uk/news/worldnews/southamerica/brazil/4307262/Nazi-angel-of-death-Josef-Mengele-created-twin-town-in-Brazil.html
 | title = Nazi Angel of death Josef Mengel created twin town in Brazil
 | author = Nick Evans
 | date =21 January 2009
 | publisher = The Telegraph
}}</ref> The Argentinian historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Joseph Mengele could be responsible for the high ratio of twins in the area.<ref>{{cite news
 | url = http://www.thesun.co.uk/sol/homepage/woman/real_life/article2162918.ece
 | title = The twins from Brazil
 | author = Caroline Iggulden
 | date = 23 January 2009
 | publisher = The Sun}}</ref> 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.<ref>Linda Geddes: Nazi 'Angel of Death' not responsible for town of twins New Scientist online, 27 January 2009</ref> A high twinning rate has also been observed in other places of the world, including Igbo-Ora in Nigeria<ref>Template:Cite news</ref> and Kodinji in India.<ref>Template:Cite news</ref>

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/1000 births. Twenty six per cent 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.<ref name="hausa">Template:Cite journal</ref>

===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.*She has a family history of dizygotic twinning, especially a mother who is a twin.Template:Fact

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 fertilisation (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, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.Template:Fact

==Delivery interval==A 15-year German study<ref name="acta">"Twin-to-twin delivery time interval: influencing factors and effect on short-term outcome of the second twin"; Werner Stein, Björn Misselwitz & Stefan Schmidt; Acta Obstetricia et Gynecologica Scandinavica, 87(3):346-353, 2008.</ref> of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.<ref>There were 836,104 deliveries of babies in Hesse over the 15-year study period, including 11,740 twin pregnancies, of which only 4,110 met the inclusion criteria and hence were examined in the study.  The excluded twin pregnancies were in cases of (1) delivery before 34 weeks of gestation; (2) when the first twin was delivered by caesarean section; (3) when either of the twins had died in the womb before the onset of labor; and (4) when the pregnancy had been complicated by fetal malformations or Twin-to-twin transfusion syndrome.</ref>  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=

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 has died very early in the pregnancy and has not been detected or recorded.<ref name="Keith-2">{{cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|first=Luigi
|last=Gedda
|editor=Louis G. Keith, Emile Papiernik, Donald M. Keith and Barbara Luke
|isbn=1-85070-666-2
|publisher=Taylor and Francis
|chapter=The role of research in twin medicine
|year=1995
|pages=4}}</ref> 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.

File:Chang-eng-bunker-PD.gif

Conjoined twins

Main article: Conjoined twin

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<ref name=Shulman>Template:Cite book</ref> 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===

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 colouration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin.In addition, in certain cases the person or chimera may have two sets of DNA.

Parasitic twins

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 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. Throughout their lives twins tend to be smaller than singletons on average.

===Twin-to-twin transfusion syndrome===

Main article: Twin-to-twin transfusion syndromeMonozygotic 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

Main article: Twin study

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 largely been replaced in favor of modern molecular genetic methodologies.

===Unusual twinnings===There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.Template:Fact

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.<ref>Template:Cite news</ref>

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. Heterotopic pregnancy occurs at a rate of about one in 30,000 pregnancies.Template:Fact

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases<ref>Template:Cite journal</ref>) 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===Template:Seealso

Monozygotic twins can develop differently, due to different genes being activated.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite news</ref><ref>Template:Cite journal</ref>

File:Hereford twins.jpg

==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%, though twinning is unfavorable, as inter-sex twins are sterile and found to have poor quality carcass at processing. The nine-banded armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.<ref>Cyranoski D. (2009). Developmental biology: Two by two. Nature. 458(7240):826-9.Template:PMID</ref><ref>Newman HH. Patterson JT. (1909). A Case of Normal Identical Quadruplets in the Nine-Banded Armadillo, and Its Bearing on the Problems of Identical Twins and of Sex Determination. Biological Bulletin, 17: 181-187 Template:JSTOR</ref>

==See also==
Template:Commonscat
*Evil twin
*Gemini
*Incest between twins
*List of multiple births
*List of twins
*Look-alike
*Litter (animal)
*Multiple birth*Caesar Twins

== References ==

== Further reading ==
* Template:Cite journal
* Template:Cite journal
* Template:Cite journal
* Template:Cite journal
* Template:Cite journal
* Template:Cite book
* Template:Cite journal
* Template:Cite web* Template:Cite web

Template:Twin conditions


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