Data based on total births from a geographically defined population with zygosity determined from blood samples and placentation and with data on the use of oral contraceptives routinely collected in early pregnancy showed no association between oral contraceptive use prior to pregnancy in either MZ or DZ twinning. Three mutually exclusive control groups of singletons were used to take account of age, parity and secular trends.
PIP: A review of total births to women in the Aberdeen City District (Scotland) between 1969-83, with zygosity determined from blood samples and placentation and with data on oral contraceptive (OC) use routinely collected in early pregnancy, failed to reveal an association between OC use prior to pregnancy and either monozygotic (MZ) or dizygotic (DZ) twinning. Previous researchers have suggested that the recent decline in DZ twinning may be attributable to OC use. For the 384 twin births that occurred during the 15-year study period, a randomized selection procedure was adopted to produce matches of equiprobability from the more than 41,000 singleton births recorded in the district. 3 matched singleton control mothers were identified for each mother of twins. After matching for year of birth, 3 separate control groups were matched for age only, for parity only, and for age and parity. The percentage of OC users in the 3 singleton control groups during the study period was found to not differ statistically from that for mothers of twins: age-matched control group, 61.2%; parity-matched control group, 58.1%; age- and parity- matched control group, 64.6%; and twin births, 62.2%. In addition, there were no significant differences in rates of OC use between mothers who gave birth to MZ twins versus DZ twins. Also investigated was the possible significance of the interval between cessation of OC use and pregnancy for twinning. Matched pair analyses were conducted for pregnancy within 2, 3, 6, and 12 months of discontinuation of OC use. None of the results achieved statistical significance. Although the present study failed to confirm an association between OC use and either MZ or DZ twinning, it should be noted that information was not available on the type of OC used.