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Triplets occur where three children are born in a multiple birth.
Multiple pregnancies in humans are invariably born prior to 40 weeks of gestation, the average length of pregnancy. is 34 weeks for triplets.
In general, twins occur naturally at approximately the rate of 1/89 of singleton births, triplets at 1/89 the rate of twin births, and so on (Hellin's Law). In North America, dizygotic twinning occurs about once in 83 conceptions, and triplets about once in 8000 conceptions.