The male disadvantage and the seasonal rhythm of sex ratio at the time of conception

BACKGROUND: In accordance with a presumed greater fragility of male versus female pregnancies, we tested whether sex ratio (male/female ratio) of vital pregnancies is higher in seasons more favourable for reproduction. METHODS: A retrospective study was performed on 14 310 births which had occurred in our institute between 1995-2001. For each single pregnancy the time of conception was calculated by the last menstrual period recall and confirmed or redefined by ultrasound in 95.8% of cases. The sex ratio of 199 454 pregnancies which had occurred in the Modena County between 1936-1998 was also stratified according to the month of birth. RESULTS: Sex ratio of institutional deliveries was 0.511 and was identical to that obtained from the County registry. Sex ratio at birth did not show a significant seasonal variation. By contrast, sex ratio calculated at time of conception showed a seasonal rhythm, with amplitude of 2.4% and peak values in October (confidence interval: 43 days). The rhythm was in phase with the rhythm of conception that showed peak values in September (confidence interval: 37 days) and an amplitude of 7%. CONCLUSIONS: The superimposition of the phase of sex ratio and conception rhythms supports the contention that more males than females are conceived in seasons with more favourable reproductive conditions.