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) 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-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, and in 0.3% of all pregnancies. monochorionic-diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. 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. monoamniotic twins are always monozygotic. the survival rate for monoamniotic twins is somewhere between 50% to 60%. 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. conjoined twins 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 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 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. ethnicity 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. 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 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. a high twinning rate has also been observed in other places of the world, including: igbo-ora in nigeria kodinhi, 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. twins are more common in people of african descent. 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. 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 pregnancy. 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% 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. complications during 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 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. conjoined twins 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 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 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, lydia fairchild, 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 twin. 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 treated medically. 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, as the twin is not reabsorbed. 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. twin-to-twin transfusion syndrome main article: twin-to-twin transfusion syndrome twin-to-twin transfusion syndrome (ttts) illustration of twins showing one fetus with exposure to more amniotic fluid while the other is "stuck" with the membrane tightly around itself. 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. stillbirths stillbirths occurs when a fetus dies after 20 weeks of gestation. there are two