Conjoined twins are a rare event in the world’s delivery rooms. They occur about once in every 50,000 births, but 40% are stillborn, and, curiously, 70% are female. Conjoined twins are always identical: the product of a single egg that for some unknown reason failed to divide fully into separate twins during the first three weeks of gestation. In the U.S. there are perhaps 40 live cases each year; ordinary identical twins are 400 times as common.
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