Why Do Birth Rates Peak at Different Times in Different Places?

The state of the universe.
April 28 2014 1:04 PM

A Time to Be Born

Why do birth rates peak at different times in different places?

Newborn babies lay in their beds.
The farther from the equator, the earlier in the year this scene is likely to take place.

Photo by Waltraud Grubitzsch/AFP/Getty Images

The Book of Ecclesiastes proclaims that there is “a time to be born.” It seems that time might depend on where you are born. Human births surge seasonally. The phenomenon has been remarked upon since the 19th century. At the beginning of the 20th century, social scientists began documenting the periodicity in earnest, first in Europe and later in North America. A steady stream of scholarship has continued, looking for clues and patterns.

A comprehensive analysis of when people give birth was recently published in the Proceedings of the Royal Society B. Micaela Martinez-Bakker and Kevin Bakker, both of the University of Michigan, and their colleagues observed a previously unnoticed phenomenon: Peak months for births change with latitude. The most popular month for birthdays occurs earlier and earlier in the year the farther north you travel from the equator.

For the United States, the researchers looked at birth records by state from the Census Bureau dating back to 1931. They found that a surge in births happens at the same time every year for particular locations. Before the baby boom era, November was the most common month to be born in Florida. In Ohio, it was June. More recently, the top birth times for each state have shifted. Peak months in the United States are now later in the year and occur closer together, but the pattern has remained, with northern states having slightly earlier peak birth months than southern states.

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The new analysis of births covers 118 countries, mostly in the Northern Hemisphere. It revealed that this pattern holds true outside the United States. Many a French mother delivers between May and July. In French Guiana, women give birth in greatest numbers in October. Good data for countries in the Southern Hemisphere proved hard to get, but the authors suspect that peak birth times also follow a pattern of change with distance from the equator going south.

Doing the math, this means that people in more northern latitudes are conceiving successful pregnancies more often in autumn, and those farther south are doing so in winter.

So is the “time for love” when the weather gets cooler? Perhaps Canadians are doing it en masse in autumn? The reasons for successful conception are much more complex than simply the frequency of intercourse. It’s plausible that people are more likely to have sex during certain times of the year. A survey from Trojan, the condom company, suggests that Americans have more sex when the weather’s hot—which doesn’t sync with the birth rates and geographical patterns. Condom sales are sometimes used as a proxy for sexual activity, but this is problematic for explaining conception rates (for obvious reasons).

It does seem likely, however, that an increase in both protected and unprotected sex happens around Christmastime: In the past, researchers have noticed that there is both a spike in condom sales in December, and an increase in termination of unwanted pregnancies in the weeks thereafter.

Other behaviors provide clues to explain the changes in birth times with changing latitudes. There do seem to be some practical considerations people think about when planning a pregnancy, and they may contribute to the regular waves of birth. To be heavily pregnant in summer is unpleasant, and expectant parents would be wise to plan to avoid it. A recent paper from MIT suggests that seasonal birth patterns in the United States are driven by planned pregnancies, not unplanned ones.

Behavior aside, environment does seem to affect fertility in would-be mothers and fathers both. There is a regular seasonal variation in sperm quality. A recent study examined several thousand semen samples and measured sperm concentration, motility, and morphology. Sperm concentration was highest in spring and lowest in autumn. Motility was highest in summer, and it, too, decreased through the autumn months. And reproduction is, of course, extremely energy intensive. Ovarian function seems to vary with seasonal changes in energy. Successful birth depends on things going right throughout gestation, and there may be seasonal effects on miscarriage, for example from influenza outbreaks.

The truth is we just don’t know why humans are more likely to give birth at certain times in certain places. These factors and additional mysterious and inexplicable ones may all play a role. But the knowledge of when to expect a seasonal birth crest is in and of itself important. A surge in a local infant population has implications for public health: It subtly influences the nature of outbreaks of childhood diseases. When many babies are born around the same time, they become susceptible to a disease simultaneously and might thus transmit the disease more readily.

The data compiled in the new Proceedings of the Royal Society B study reveal that pulses in birth rates are extremely regular year after year. A better picture of the annual geographic timing might help spur smarter public health planning. For example, a campaign for diphtheria, polio, or hepatitis B immunization—for which vaccine doses are suppose to be given in the first six months of life—might be more effective in the months after a local surge in birth. In the United States, National Infant Immunization Week, a yearly campaign, takes place at the end of April and runs through this week. But perhaps it ought to be moved. With the exception of a few states, the peak birth months in America are not until after the summer. It is worth scrutinizing what is the right time to heal and, for the immune system, the right time to build up.

Boer Deng writes for Nature. Follow her on Twitter.