In which month are the most babies born?

More than 3.8 million babies were born each year in the United States on average from 2010 to 2020, according to the National Center for Health Statistics. But in which month are the most babies born, and why?

In the United States, the month with the largest number of births is typically August, with July a close second, Brady Hamilton, a demographer with the National Center for Health Statistics, told Live Science.

For example, August saw the greatest number of births in eight of the years from 2010 to 2020, Hamilton noted. July nabbed first place in three of those years — 2014, 2015 and 2020, he said.

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In 2021, August was again the leading month, seeing 329,978 births, compared with an average of 350,067 births from 2010 to 2020. The smallest number of births in 2021 occurred in February, which saw 266,308 births, according to the National Center for Health Statistics.

Since the full course of a pregnancy is normally about 40 weeks, or a little more than nine months, these data suggest most children in the United States are conceived in November or December, Hamilton noted. "It's a fascinating question why that is," Hamilton said.

Reproduction appears to be seasonal in many living organisms — including plants, insects, reptiles, birds and  mammals — likely to help maximize their chances of reproduction over the course of their lifetimes, Micaela Martinez, an assistant professor of environmental health sciences at Columbia University in New York, and Kevin Bakker, previously an assistant research scientist at the University of Michigan and now a principal scientist at Merck, wrote in an essay for The Conversation.

In a 2014 study in the journal Proceedings of the Royal Society B: Biological Sciences, Martinez, Bakker and their colleagues noted that there was a clear pattern of births across latitudes in the United States: Northern states have a birth peak in June and July, whereas Southern states experience a birth peak later, in October and November.

The 2014 study found a similar pattern across the Northern Hemisphere, with peaks occurring later in the year the closer the location was to the equator. For instance, Finland saw the most births in late April, whereas Jamaica's births peaked in November.

So what could explain this correlation?

A 1990 study in the Journal of Biological Rhythms suggested the seasonality of human births may be linked with local temperature and day length. The extent to which temperature and day length may or may not change seasonally over the year depends in part on latitude, Martinez and Bakker noted. These environmental changes may influence the frequency of sex or how fertile men or women are, they noted. However, in their 2014 study, they note many other factors may play a role as well, such as income, culture, holidays and rainfall, making it challenging discussing whether and in what way temperature or day length might affect human births.

Understanding when most human births happen is more than just a matter of trivia, Hamilton said.

"If you're the birthing ward of a hospital, or a company that makes baby formula, or a diaper manufacturer, it's good to know what you might be facing over time," Hamilton said. "Further down the line, school systems may want to anticipate how many children may be coming in at a certain point in time, and health care systems would want to know what to expect in terms of inoculations."

Originally published on Live Science on July 27, 2010 and rewritten on Aug. 19, 2022.