Sonographic dating of pregnancies conceived after contraceptive pill therapy.

A total of 358 women succeeded in conceiving in the first cycle after stopping oral contraception. The estimated day of delivery was calculated from both an ultrasonographically measured fetal biparietal diameter, in the second trimester, and from the date of the withdrawal bleed. According to the biparietal diameter measurement, the estimated day of delivery was postponed more than 1 week in 138 cases (38.5%). For 304 women who delivered after the spontaneous onset of labor, the biparietal diameter estimate gave the best prediction of the day of delivery in 175 cases (57.6%), the date of the withdrawal bleed gave the best estimate in 91 (29.9%) (p < 0.001) and they were equally good in 38 (12.5%). The estimated day of delivery using a biparietal diameter measurement was within +/- 14 days of the day of delivery for 267 of these 304 women (87.8%). The corresponding figure for the withdrawal bleed date was 218 (71.7%) (p