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multiple births The production of more than one individual in a single parturition. Twins occur in about one in every 84 births in Britain, and about one in 300 pregnancies results in identical twins. Multiple births may occur as a result of the near-simultaneous release and fertilization of more than one ovum, or from the fertilization of a single ovum which then separates into two or more parts from each of which a new individual develops, all having identical genetic constitution. Non-identical (dizygotic or fraternal) twins may be of the same or of different sex and are no more alike than any other two siblings. Each has his or her own PLACENTA and set of membranes. Identical (monozygotic) twins share a placenta and often compete for nourishment so that they are of unequal weight when born. Multiple pregnancies are shorter than single pregnancies. Triplets and higher number births are biologically rare. Triplets have a natural frequency of about one in 7000 births. They are more common as a result of the use of anti-infertility drugs that induce multiple ovulation.