However, in most cases, the intrauterine pregnancy can be salvaged. One study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2. These studies compare monozygotic and dizygotic twins for medical , genetic , or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. This phenomenon is known as heteropaternal superfecundation. This occurrence is similar to the vanishing twin syndrome, but typically occurs later, as the twin is not reabsorbed. 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. Another abnormality that can result in monozygotic twins of different genders is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated.
These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes. 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. There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. In one case DNA tests determined that a woman, 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. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language. Parasitic twin Sometimes one twin foetus will fail to develop completely and continue to cause problems for its surviving twin. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time. 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 degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain , heart or liver. One twin, the 'donor' twin, is small and anemic , the other, the 'recipient' twin, is large and polycythemic. 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. This occurrence is similar to the vanishing twin syndrome, but typically occurs later, as the twin is not reabsorbed. This occurs when the zygote starts to split after day 12  following fertilization and fails to separate completely. Dizygotic twins from biracial couples can sometimes be mixed twins , which exhibit differing ethnic and racial features. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic. Intrauterine death occurs when a baby dies during late pregnancy. This condition occurs in about 1 in 50, human pregnancies. More than half of twins are born weighing less than 5. This phenomenon is known as heteropaternal superfecundation. This form had been speculated until only recently being recorded in western medicine. Partial molar twins[ edit ] 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 can lead to the possibility of a woman carrying fraternal twins with different fathers that is, half-siblings. Twin-to-twin transfusion syndrome[ edit ] Main article: Miscarried twin[ edit ] 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 results in one normal male XY and one female with Turner syndrome 45,X.
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