Individual data on menstrual cycles of noncontracepting women living in Western countries were used in order to verify whether the biological seasonality of conception persists after sexual behavior is controlled for. Episodes of intercourse were recorded daily, and the time of ovulation was detected by a marker. We find that the seasonality of conception changes with woman’s age and frequency of episodes of sexual intercourse. In particular, for women aged 27–31 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 in the Northern Hemisphere, if seasonality of acts of sexual intercourse is controlled, the monthly distribution of probability of conception is bimodal, with two maxima (September and January) and two minima (December and March). When unobserved characteristics of the couples are considered, this seasonal pattern of conception persists.

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