Birthday cake

Birthday cake. Photograph by Will Clayton

Both my sons were born in September. One was conceived just days after I decided to get pregnant; the other took five months. I joked that my body must like getting pregnant in January.

Joking aside, there might be something to that.

It turns out, my boys are in good company – September is a banner month for birthdays. When the New York Times published a birthday month frequency list, it went viral when Matt Stile visualized it as a heat map.* As you can see from the visualization, September’s a big month for births. My boys are in good company. How does your birthday rate?

How popular is your birthday?

How popular is your birthday? Heat map by Matt Stiles at the Daily Viz

The graph makes clear that birthdays don’t happen randomly. Lots of babies are born in the fall, but the question is, why? Are our bodies trying to time births for what was once an autumn harvest season, or do we have lots of sex when we’re snowed in over winter break?

Happy You’re-No-Longer-Independent Day

More on the conception issue in a minute. First, see that hole in the data in early July? That’s right, fewer babies are born on the 4th (and 5th) of July than on other days that month. That’s a hint that the data come from US births. You might know that the 4this a big holiday in the United States.But how does a fetus know it’s a holiday?

What we’re seeing here is in part an effect of scheduled births. Imagine a mom-to-be on the phone, scheduling an induction or a Caesarian section: “Oh, not the 4th … how about the following Monday?” Or an obstetrician: “I’m taking the 4th off, don’t schedule anything that day.”

There’s also a local peak on Feb 14. Perhaps Valentine’s Day sounds like a good birthday to that hypothetical mom on the phone. And a lot of superstitious people might actively avoid the 13th of any month to escape giving their child an ‘unlucky’ number for a birthday.

Does scheduling delivery really affect annual birth patterns? Actually, yes – the fact that 22% of labors are induced means selecting specific days for a birth, and those choices tend to be weekdays. In fact, there is a strong “weekday bias” in births that dates back to the 1930s, when the trend for having babies in hospitals started to take off. These days, almost twice as many babies are born on Monday than Sunday.

This weekday bias spotlights a trend that some argue is harmful: scheduling births for the doctor’s convenience, when allowing labor to proceed naturally might be a better choice. This bias doesn’t exist when women labor without intervention, according to a study that compared births in Japanese hospitals with those in midwife-attended maternity centers, where no surgery or medical induction occurs. The birth rate in the maternity centers is a flat line across the week while the hospital births plummet on weekends and holidays.

‘Tis the Season

These modern medical shifts might explain weekday frequencies, but what about all those late summer babies? Do humans really have a ‘breeding season’?

The graph (right) shows total births by month. You can see that plenty of babies are born year-round, but there is a definite “hump” in the data (sorry) that peaks in August.

Several factors might be at work here.

One popular explanation is that it’s there is a “Christmas effect” resulting from either holiday merriment or being snowed in with nothing better to do. Both explanations have their weaknesses. For example, does your snow/holiday season start around October … or ever? Plus, I’d expect the major sexy holiday to be Valentine’s Day — and yet there is no uptick in births in early November. But this is, in fact, a testable hypothesis: What happens in other countries and climates?

Data from northern Europe show a strong seasonality trend, and this is a region that has a major winter holiday and plenty of snow. The problem is, conceptions there are lowest in winter — and they correlate inversely with the darkest days, not the holiday season. This pattern puts the major peak of births in spring; much of Europe has this spring peak, in contrast to the autumn peak in the US. But since 1970 or so, the pattern in Europe has been shifting, with autumn becoming the major peak. The reason isn’t clear. One paper suggests that partners who are separated might get together for the holidays. Another points out that decreasing spring conceptions could correspond with a declining trend in the European tradition of August vacations.

Indeed, as one biologist wrote in referencing the European data: “If [hedonism in festival times such as Christmas]were the sole explanation of the autumn peak in births, one would have to interpret the USA as being more hedonistic than Europe across the years 1850-1950. The novels of Henry James and Edith Wharton suggest otherwise.” Of course, they’d suggest that only for the upper middle classes, who weren’t the only people having children during that time.

But more than cultural factors drive this seasonality in birth. In nonhuman animals, the amount of light each day, or photoperiod, underlies seasonal cycles, including birth. If we evolved to be more fertile at certain times, chances are good that photoperiod would be the signal for that timing — and, in fact, the data seem to agree. Birth seasonality is strongest at extreme latitudes where photoperiod is most extreme, and an Italian study found that day length and temperature (which are hard to separate) explained 40%of the variability in seasons of birth.

Why would there be a season at all? Some monkey species bear young just as food is becoming more available, so the mothers can take in plenty of calories while they’re lactating. In other species, the food peak comes at weaning time instead, when the young need to eat on their own. It’s possible food availability also once drove human birth patterns. Today, subsistence farmers in Zaire experience extreme differences in food availability depending on season, but their neighbors who forage for food can find it more or less year-round. Both births and ovulation show a strong seasonal pattern among the agriculture-based communities — a clue that conception and birth don’t rely only on when people have sex, although sexual activity could be seasonal, too, as the “Christmas effect” would suggest.

In spite of these subtle rhythms, babies still arrive year-round. In some parts of the world, year-round lighting thanks to electricity might explain why births aren’t as seasonal as they used to be. Better year-round access to food also could be a factor, as women with ready caloric access at any time don’t necessarily need to reserve ovulation for seasons of abundance. Some researchers propose that only some of us show a response to day length. Women who were born during peak times also tend to give birth during peak times, suggesting genes could play some role.

Some of us might still retain the machinery for registering cues from nature about the optimal time for baby-making. And in addition to nature’s cues, people also obviously incorporate cultural, social, fiscal, and personal physical factors into the natural rhythms that might guide us. Given all of the possible influences on timing of conception and birth, I will probably never know if my two September babies were coincidence or something more coordinated … but at least they’re in good company.

Written by Beth Skwarecki

Beth Skwarecki is a science writer based in Pittsburgh, PA. Find her on twitter: @BethSkw


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