Arnaud Régnier-Loilier

Both the shape and magnitude of seasonal patterns of birth in France have changed in recent years. Birth seasonality is less pronounced today that it was in the past, and the increase in May births observed from 1975 to 1980 has disappeared. However, we do find one invariant every year: a slight peak in births spanning several days around September 25. Taking into account pregnancy length distribution (the average pregnancy length is 268 days, depending on pregnancies), that peak corresponds to New Year’s Day conceptions.

Coverage: all of France
Source: INSEE, 2018 birth records (live births)
Reading: An index of 105 (respectively 95) indicates a day on which there were 5% more (respectively 5% fewer) births than on an average day (base 100 for the year 2018)
Note: Base index = 100; the red curve indicates centered moving averages over 21 days.

Though the September peak is quite remarkable at the scale of a single year, it is not very intense. Whereas in 2018 the average number of recorded daily births in France was 2,078, the number rose to 2,233 for the week of Saturday, September 22 to Friday, September 28 (7% more births). At the scale of a single maternity hospital, then, the surplus can barely be perceived. For example, a medium-sized maternity hospital, delivering 1,500 babies a year, the equivalent of 28 deliveries per week on average, would have recorded for that week in September only two deliveries more than for an ordinary week.

There are two ways of explaining the late-September peak in births. Either we can assume that a greater proportion of couples wishing to have a baby—and therefore not using contraception—have sexual relations the night of December 31. We do not have any precise data on the matter, though. Or we can interpret the September peak as an effect of reduced contraceptive vigilance on New Year’s Eve. This hypothesis is supported by abortion statistics, which show a considerable “excess” of abortions corresponding to conception on or around January 1st.

Related articles:

Breton D., Barbieri M., d’Albis H., Mazuy M., 2018, “Recent Demographic Developments in France: Seasonal Patterns of Births, Deaths, Unions, and Migration”, Population - E : English edition, 73(4): 591-658.

Leridon H., 1986, « Les conceptions du 1er janvier (ou: les étrennes de septembre) », Population, 41(3): 599-602.

Régnier-Loilier A., 2010, “Changes in the Seasonality of Births in France from 1975 to the Present”, Population - E : English edition, 65(1): 145-185.

Régnier-Loilier A., Rohrbasser J.-M., 2011, “Is there a childbearing season?”, Population & Societies, 474.