Oral contraceptives postdatism as a complication.

The relationship between previous use of oral contraceptives (OCs) and postdatism was investigated in 1496 women attending an antenatal clinic in a 7-month period in 1981. Postdatism was defined as spontaneous labor and delivery 14 or more days beyond the expected date of delivery as calculated by Naegele's rule (i.e., 9 months plus 1 week from the last menstrual period). 186 of the women reported OC use in the period before conception of the present pregnancy and 37 of their deliveries were postdate, for a rate of 19.9%. There were 65 postdate cases among the 1310 nonusers, for a rate of 5%. Among OC users, 43% of the postdatism was 28 days beyond term and 21.6% was 35 days beyond term. The corresponding percentages for nonusers were 27.7% and 10.7%. This indicates that not only is there 5 times more postdatism among women who use OCs before conception than among nonusers, but the postdatism is more severe in the former group. Although no association was found between the duration of OC use and postdatism, there was a clear correlation with the length of the OC cessation-conception interval. Women who conceived 1 month after discontinuing OC use demonstrated a 28.5% rate of postdatism. The rates for 1-2, 2-3, and 3-4 months from cessation to conception wer 27%, 25%, and 16%, respectively. Overall, the postdatism rate was 25.7% among women who conceived in the 1st 4 months since cessation of OC use, compared with a rate of 12.3% among women in whom the cessation-conception interval was over 4 months. Routine induction at 14 days beyond term is not recommended for women with a history of recent OC use. This would result in higher rates of induction for this group than for nonusers, more cesarean sections, and increased neonatal mortality due to prematurity. It is concluded that calculation of the expected date of delivery by use of Naegele's rule is not appropriate in many women with a history of recent OC use. It is suggested that clinicians rely on ripeness of the cervix, the date of the 1st fetal movements, fundal heights, and ultrasonography to assess the need for induction.