Over the past thirty years, the seasonal pattern of births in France has evolved gradually, with a loss in amplitude and a shift of the mode from May to September. The reasons for these changes remain difficult to determine. Nonetheless, exhaustive data from civil records combined with abortion statistics provide an explanation for the current distribution of births, with the September “peak” corresponding to a surplus of conceptions on New Year’s Eve. There are several discontinuities in the seasonal pattern, however. For instance, the major heat waves observed since 1975 have led to fewer deliveries nine months later. The decrease in births observed in late summer 1975 may have been the result of the Veil Act legalizing abortion. This interpretation must be taken with caution, however, as the effect was short-lived.