The Seasonality of conception

Individual data on menstrual cycles of non-contraceptive women living in western countries were used in order to verify if the biological seasonality of conception persists after controlling for sexual behaviour. Episodes of intercourse were recorded daily, and the time of ovulation detected by a marker. We find that the seasonality of conception changes with a woman’s age and number of acts of intercourse. In particular, for women aged 26-|31 having only one act of intercourse during the six most fertile days of the menstrual cycle, the seasonality of fecundability is stronger. In this age group, the monthly distribution of probability of conception is bi-modal, with two maxima (September and January) and two minima (December and March). When unobserved fixed variables characterizing each woman are considered, the seasonality of conception remains.