twin twin 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 ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. in rare cases twins can have the same mother and different fathers (heteropaternal superfecundation). identical twins mark and scott kelly, both former nasa astronauts in contrast, a fetus that develops alone in the womb (the much more common case, in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple. unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgängers. statistics the human twin birth rate in the united states rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. the yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (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 south america, south asia (india, pakistan, bangladesh, nepal), 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. some women intentionally turn to fertility drugs in order to conceive twins. types and zygosity the vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). less common variants are discussed further down the article. fraternal twins can be any of the following: female–female twins: sometimes called sororal twins (25%). male–male twins: sometimes called fraternal twins (25%). female-male twins: this is the most common pairing (50%), encompassing both "female-male" (25%) and "male-female" (25%) twins. 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. common name scientific name zygosity development occurrence identification health other identicalmonozygoticxxxxxx fraternaldizygoticxxxxxx half-identicalsesquizygoticxxxxxx mirror imagexxxxxxx mixed chromosomexxxxxxx superfecundationxxeggs are fertilized during different acts of intercoursexxxusage 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 fathers. superfetationxxa female gets pregnant again while already pregnant, resulting in multiple fetuses at differing developmental stagesxxxx parasitic twinxxxxxby definition only healthy fully formed fetusx vanishing twintwin resorption, twin embolisation syndromexxup to 1 of every 8 multifetus pregnanciesxby definition only healthy fully formed fetuschimerism, mosaicism polar bodyxxxxxxx conjoined twinxxxxxranges from normal to compromisedx dizygotic (fraternal) twins adult fraternal twins ibeyi duo naomi (left) and lisa-kaindé diaz are dizygotic (fraternal) twins. two fraternal twin brothers have commonalities. fraternal twin brothers as young babies. 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 develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings. this is the most common type of twin. dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis. dizygotic twins share only 50 percent of each other's genes, which resemble amongst siblings that are conceived and born at different times. like any other siblings, dizygotic twins may look similar, particularly as they are the same age. however, dizygotic twins may also look very different from each other (for example, be of opposite sexes). 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 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. monozygotic (identical) twins 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. the chances of having identical twins is relatively rare — around 3 or 4 in every 1,000 births. mechanism regarding spontaneous or natural monozygotic twinning, a recent theory proposes that monozygotic twins are probably formed when a blastocyst contains two inner cell masses (icm), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the zona pellucida (the gelatinous protective coating around the blastocyst). 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 transfer. incidence monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide. 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 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 twins are genetically nearly identical and they are the same chromosomal 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 the same genotype. 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 unevenly. 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 12 million 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 their lives. 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. monozygotic twins 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. in january 2021, new research from a team of researchers in iceland was published in the journal nature genetics suggesting that identical twins may not be quite as identical as previously thought. the four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development. 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 body twinning. however, a study in 2012 found that it is possible for a polar body to result in a healthy fetus. in 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) 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 automatically implies dizygotic twins,