In the U.S., birthdays peak in early September, but in Northern states — and Scandinavia — the peak comes earlier, in the summer or spring.
Though many factors likely go into the popularity of birthday months (a spike in November is often attributed to Valentine’s Day), seasons themselves may play a role in how easy it is to conceive, according to a new Boston University School of Public Health study.
The first-of-its-kind study, published in the journal Human Reproduction, finds that, although couples in North America and Denmark are most likely to start trying in September, it’s in late November and early December that they have the best chances of conceiving, especially at lower latitudes.
“There are a lot of studies that look at seasonal patterns in births, but they don’t take into account when couples start trying, how long they take to conceive, or how long their pregnancies last,” says study lead author Dr. Amelia Wesselink, postdoctoral associate in epidemiology.
“After accounting for seasonal patterns in when couples start trying to conceive, we found a decline in fecundability in the late spring and a peak in the late fall,” she says. “Interestingly, the association was stronger among couples living at lower latitudes.”
North Americans were more likely than Danes to begin trying to conceive in the fall. But, after taking those patterns into account, season affected fecundability for North Americans by 16 percent, while Danes had an 8-percent seasonal boost in the fall and dip in the spring. In Southern U.S. states, seasonal variation was stronger, at 45 percent, with a peak in conceptions in late November. Meanwhile, the relationship between season and fecundability was similar in Denmark and Northern states and Canada.Friday Letter Submission, Publish on February 28