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rtilization resulting in monozygotic twins is uniformly distributed in all populations around the world 24 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 28 and up to over 20 in some central african countries 11 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 29 genetic and epigenetic similarity edit 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 30 monozygotic twins can have rare differences in their genotypes normally due to an environmental factor or the deactivation of different x chromosomes in female monozygotic twins in some extremely rare cases due to aneuploidy twins may express different sexual phenotypes typically from an xxy klinefelter syndrome zygote splitting unevenly 31 32 33 to quantify the occurrence of such genotypic differences a 2013 study of dna in white blood cells of 66 pairs of monozygotic twins tested 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 34 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 citation needed despite being genetically identical twins mark and scott kelly are distinguishable from each other 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 35 however certain characteristics become more alike as twins age such as iq and personality 36 37 38 a four year study of monozygotic twins and their extended families published in nature genetics in 2021 found that these twins have genetic differences that begin in the early stages of embryonic development 39 40 polar body and semi identical twins edit a 1981 study of a deceased 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 41 however a study in 2012 found that it is possible for a polar body to result in a healthy fetus 42 in 2003 a study argued that many cases of triploidy arise from sesquizygotic semi identical twinning which happens when a single egg is fertilized by two sperm and splits the three sets of chromosomes into two separate cell sets 43 44 degree of separation edit various types of chorionicity and amniosity how the fetus sac looks in monozygotic 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 four days monoamnionic twins divide after the first week citation needed 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 45 furthermore there can be various degrees of shared environment of twins in the womb potentially leading to pregnancy complications citation needed it is a common misconception that two placentas automatically implies dizygotic twins but if monozygotic twins separate early enough the arrangement of sacs and placentas in utero is in fact indistinguishable from that of dizygotic twins type description day 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 46 and in 18 36 47 or around 25 46 of monozygotic identical twins didi twins have the lowest mortality risk at about nine percent although that is still significantly higher than that of singletons 48 dichorionic diamniotic twins form when splitting takes place by the third day after fertilisation 46 monochorionic diamniotic 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 47 and in 0 3 of all pregnancies 49 monochorionic diamniotic twins are almost always monozygotic with a few exceptions where the blastocysts have fused 46 monochorionic twins share the same placenta and thus have a risk of twin to twin transfusion syndrome days 4 8 monochorionic monoamniotic monochorionic twins share the same amnion in 1 2 of monozygotic twin pregnancies 47 monoamniotic twins are always monozygotic 50 the survival rate for monoamniotic twins is somewhere between 50 50 and 60 51 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 develop cerebral palsy due to lack of oxygen monoamniotic twins occur when the split takes place after the ninth day after fertilization 46 conjoined twins when the division of the developing zygote into two embryos occurs 99 of the time it is within eight 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 eight weeks and five 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 rightwards 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 demographics edit 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 52 from 1980 to 1997 the number of twin births in the united states rose 52 53 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 52 ethnicity edit main article populated places with highest incidence of multiple birth a pair of female ere ibeji twin figures early 20th century in the permanent collection of the children s museum of indianapolis the yoruba people have the highest dizygotic twinning rate in the world about 1 in 90 human births 1 1 results from a twin pregnancy 54 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 55 in cândido godói one in five pregnancies has resulted in twins 56 the argentine historian jorge camarasa has put forward the 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 57 a high twinning rate has also been observed in other places of the world including igbo ora in nigeria 58 kodinhi located in kerala india 59 mohammadpur umri located in uttar pradesh india 60 61 62 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 63 64 twins are more common in people of african descent 65 predisposing factors edit 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 66 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 pregnancy delivery interval edit a 15 year german study 67 of 8 220 vaginally delivered twins that is 4 110 pregnancies in hesse yielded a mean delivery time interval of 13 5 minutes 68 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 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 there have also been cases in which twins are born a number of days apart possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in cologne germany the first of which was born on november 17 2018 69 complications during pregnancy edit vanishing twins edit 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 70 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 conjoined twins edit main article conjoined twins chang and eng bunker born in siam now thailand in 1811 were the origin of the term siamese twins conjoined twins or the once commonly used term siamese are monozygotic twins whose bodies are joined during pregnancy this occurs when the zygote starts to split after day 12 46 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 liver or lungs chimerism edit main article chimera genetics a chimera is an ordinary person or anima...