researchers suspect many 1 in 8 pregnancies start out multiples, single fetus brought full term, because other fetus has died in pregnancy , has not been detected or recorded. obstetric ultrasonography exams reveal fetus, fails develop , instead disintegrates , vanishes in uterus. there several reasons vanishing fetus, including being embodied or absorbed other fetus, placenta or mother. known vanishing twin syndrome. also, in unknown proportion of cases, 2 zygotes may fuse after fertilization, resulting in single chimeric embryo, and, later, fetus.
chang , eng bunker, born in siam (now thailand) in 1811, origin of term siamese twins .
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