Humans do not normally produce litters. Nevertheless, it is estimated that one person in 20 who was born alone has a lost twin who was conceived at the same time, but failed to reach term.rnIt has been known for many years that the loss of one twin in this way late in a pregnancy is bad for the other. That other is more likely to be born prematurely, to have cerebral palsy, or even to die as well. What has not been known until now is whether such a loss in the first few months has any effect on the survivor. But a paper just published in Human Reproduction by Peter Pharoah of the University of Liverpool, in Britain, suggests that it does.rnThere are several ways that a "vanished twin" can come to light. Most gruesomely, fully formed body parts of the dead twin may be found embedded in the body of its surviving sibling. More commonly, lost twins are discovered as tiny mummified attachments to the placenta of the live twin, after it is born. And the widespread use of ultrasonic scanning means that, increasingly often, twins are spotted in early scans and subsequently vanish. Neither early ultrasonic "sightings" nor papery mummified corpses are always officially registered. They may not even be mentioned to the mother, so it is hard to know just how often a twin goes missing.
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