A Time to be Born Varies by Latitude

What's your latitude? It turns out the answer to that question will influence the likelihood of when babies will be born in your area. Boer Deng of Slate discusses an article published today in the Proceedings of the Royal Society B: Biological Sciences (which is behind a paid subscription). 

A comprehensive analysis of when people give birth was recently published in theProceedings 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.

The new analysis of births covers 118 countries, mostly in the Northern Hemisphere. It revealed that this pattern holds true outside the United States...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.

Deng notes that we aren't sure why these patterns exist, but they obviously are related to patterns of sexual activity nine months earlier. Nonetheless, the knowledge of seasonal clusters has useful health policy implications, in terms of knowing, for example, when to set up the most effective immunization campaigns.