Why Is September the Most Popular Birthday Month in the U.S.? People Have Theories

Maybe you’ve come across one or two people over the years who share your birthday. If you were born in September, that number might be a little higher.

1. The September Baby Boom
2. Potential Explanations for Why So Many Babies Are Born in December

The September Baby Boom

In 2016, data journalist Matt Stiles analyzed the number of U.S. births by day from 1994 through 2014 and found that September is the busiest month to bring home a bundle of joy. Based on his findings, September 9 is the most popular birthday overall, with 12,301 births on average, and eight other September dates surpassed an average of 12,000 births, too.

So why exactly is September such a high-traffic time to experience the miracle of childbirth? Flipping through the calendar could give us a hint.

Pregnancies are supposed to last 38 weeks from conception (or 40 weeks from a person’s last menstrual period), which means that babies born on September 9 would’ve been conceived on December 17. The conception date for the second most popular birthday, September 19, should technically be December 27.

Of course, babies don’t always decide to make their appearance precisely at the 38-week mark, but the gist is that September babies were conceived in December. As Motherly points out, people tend to work less during the holidays, leaving more time to bounce from party to party, drink a few extra cocktails, and further the family line. It’s also a romantic season in general; according to a 2018 survey by Brides, December is the most popular month to get engaged.

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Potential Explanations for Why So Many Babies Are Born in December

Scientists have proposed their own hypotheses to explain the high conception rates in December, but they’re still mostly speculation. One theory suggests that sperm are less potent during the summer; according to another, the mother’s eggs or uterine lining is less receptive to sperm during those months. Others think shorter daylight hours during the winter could affect our hormone functions.

It’s also possible that some people simply think September is an opportune time to have kids. As Experian explains, there aren’t any huge holidays to celebrate, so you can plan an induced or elective birth without worrying about short-staffed hospitals.

Whatever the reason, if you’re tired of having to schedule your birthday shindig around all the other ones in September, you can always throw yourself a half-birthday party, instead—far fewer people were born in March.

A version of this story originally ran in 2024; it has been updated for 2025.