Birth numbers vary between seasons even nowadays and in industrialised countries, where food is ample and contraceptives available; even if they do not always conform to the pre-industrial peaks in spring and autumn. For example, birth numbers in the southern US (Lousiana, Georgia, Texas, Arizona) peak in September and dip in May. These differ from patterns in northern US states (New York, Washington), where the peak is observable in September and the dip in January.

In northern and central European countries, the patterns are different again, with peaks in April, a smaller peak in September and a dip in November. Interestingly, Canada's pattern is more similar to Europeans ones than to those of the US[1]. This seems to point to general phenomenon when comparing more countries: Peak months for births change with latitude[2].

Of course, there are also social factors: In Israel, numbers for births from jewish mothers peak in August and dip in April; interestingly, births from muslim mothers in israel peak in January and dip in May[3].

This seasonability is explained either with social factors influencing the frequency of intercourse or climatological factors that influence fertility.

Sources:

[1] David Lam, Jeffrey Miron: Global Patterns of Seasonal Variation in Human Fertility. Annals of the New York Academy of Science (1994)709: 9-28.

[2] Micaela Martinez-Bakker, Kevin M. Bakker, Aaron A. King, Pejman Rohani: Human Birth Seasonality: Latitudinal Gradient and Interplay with Childhood Disease Dynamics. Proceedings of the Royal Society B: Biological Sciences (2014) 281:

[3] Michael Friger, Ilana Shoham-Vardi, Kathleen Abu-Saad: Trends and seasonality in birth frequency: a comparison of Muslim and Jewish populations in southern Israel: daily time series analysis of 200 009 births, 1988–2005. Human Reproduction (2009) 24 (6): 1492-1500.