Sex of offspring of women using oral contraceptives, rhythm, and other methods of birth control around the time of conception

Sex ratios were studied in a cohort of 33,205 newborns whose mothers had been questioned in early pregnancy about contraceptive use around the time of conception. The proportion of males was 0.517 (95% confidence limits [CL], 0.512 to 0.522). In the 9279 offspring of women who had used oral contraceptives (OC) in the 5 months prior to conception the sex ratio was 0.517 (0.507 to 0.527). There are sufficient numbers available for study for us to be 95% certain that OC causes no shift in sex ratio of 1% or more. Women who conceived within 2 months of stopping the pill had a small excess of males (0.528; 95% CL, 0.510 to 0.546), but this excess was probably due to chance. OC failures were followed by a raised sex ratio in the offspring (0.543; 95% CL, 0.509 to 0.577), and this excess of males was observed consistently in subgroups of maternal age, parity, race, and education. Failures of rhythm contraception were also associated with a consistent excess of male births (0.567; 95% CL, 0.514 to 0.620). No changes were observed after failures of IUDs, barrier, or chemical methods.