Overripe ova and twinning.

Multiple births were studied in a sample of orthodox Jewesses for whom an estimate could be made of the day of ovulation and the earliest possible day of conception. The overall rate of twinning was 14.5/1,000 deliveries, and of triplets, 0.40/1,000. Twinning rates varied significantly from 11.4 in the 5,976 "early" conceptions (day -1 or earlier relative to the estimated day of ovulation) to 26.9 in the 1,498 "late" conceptions (day 0 or later). Triplets varied significantly from 0 to 2.01 in early and late conceptions, respectively, and unlike-sexed multiple sets, 2.8 and 12.8, respectively. The excess of multiple births in late conceptions was seen within different ages and origin groups, in women with different menstrual characteristics, and in those with and without treatment for anovulation. While the excess of unlike-sexed sets seems to lead to the conclusion that late conceptions are associated with dizygosity, polar body twinning and uniovular dispermatic twinning should also be considered.