Sex ratio and time to pregnancy: analysis of four large European population surveys

Objective To test whether the secondary sex ratio (proportion of male births) is associated with time to pregnancy, a marker of fertility. Design Analysis of four large population surveys. Setting Denmark and the United Kingdom. Participants 49 506 pregnancies. Main outcome measure Secondary sex ratio. Results No association was found between the sex ratio and time to pregnancy and no discernible trend was found for sex ratio with time to pregnancy, either within individual datasets or in the pooled analysis. The odds ratios were 0.97 (95% confidence interval 0.90 to 1.04) for contraceptive failures, 1.01 (0.96 to 1.05) for time to pregnancy of 2-4 months, 1.02 (0.97 to 1.08) for 5-10 months, 0.98 (0.93 to 1.03) for 11 months or more, and 0.88 (0.74 to 1.06) for fertility treatment, with 0-1 months as the reference category. Conclusion No association was found between the secondary sex ratio and time to pregnancy.