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Birth data on 1050 sets of triplets delivered in the United States from 1985 to 1988 were analyzed to establish whether seasonal variations in the number of triplet births occur. These data were compared to live birth data from the entire United States population over a similar period; the US data exhibit a seasonal variation with a peak in late summer. Seasonality in the number of triplet births was noted, and it differed significantly (p = 0.01) from that of the entire US population. A large peak in triplet births was seen in the spring (April-May) and a smaller peak in late summer (August-September). Only minor differences in these trends could be observed when triplet data were stratified by cause of pregnancy (spontaneous versus drug-induced ovulation) or corrected for the shorter gestations of triplet pregnancies.