<P> 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 . </P> <P> Conjoined twins (or the once - commonly used term "siamese") are monozygotic twins whose bodies are joined together 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 or liver . </P> <P> 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, 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 . </P> <P> 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 . </P>

Who has the highest rate of dz twins