Teratogenic hazards of oral contraceptives analyzed in a national malformation register.

During the period 1967-1976, prospectively collected data were available in the Finnish Register of Congenital Malformations on the contraceptive usage of 3,002 mothers of children with malformations detected at birth, as well as of time-matched and place-matched control mothers. The distributions of the various types of malformations were similar among contraceptive users (immediately prior to or during the pregnancy) and nonusers. The total incidences of various methods of contraception were also similar among mothers of malformed children and the control mothers. The risk ratio method for matched pairs allowed us to evaluate the strength of the observed negative correlation concerning oral contraceptives. Thus, we could exclude with 95% confidence any effect larger than 5% of oral contraceptives on the incidence of visible malformations.