Twinning following oral contraceptive discontinuation.

The characteristics of 673 mothers of twins were compared to those of a matched sample of mothers of singletons. Both groups of mothers were interviewed just after delivery, and births in each group were matched for time and place. Of the characteristics studied, the use of oral contraceptives (OC) displayed a significant negative association with dizygotic twinning, with an estimated relative risk of 0.55 (95% confidence limits: 0.39/0.78). Among other maternal trails significantly associated with dizygotic twinning, ony age, parity and weight were possible confounders as far as the relationship between OC use and dizygotic twinning was concerned. Adjustment for these 3 characteristics left this relationship unaltered. These findings are compatible with the existence of the a direct relationship between OC use and a reduction in dizygotic twinning. Review of the available evidence concerning reproductive capacities following OC discontinuation suggests that the higher incidence of chromosomic abnormalities among spontaneous abortuses of OC users or their lower fertility could explain a reduction of dizygotic twinning rate after OC discontinuation. It is concluded that examination of te time and place variations in OC use and dizygotic twinning would help to clarify the nature of their relationship.