from the how-many-september-births-are-christmas-presents? dept.
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 out there that look at seasonal patterns in births, but these studies 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 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. ("Fecundability" refers to the odds of conceiving within one menstrual cycle.) "Interestingly, the association was stronger among couples living at lower latitudes."
[...] season affected fecundability for North Americans by 16 percent, while Danes got only an 8-percent seasonal boost in the fall and dip in the spring. In Southern US states, the seasonal variation was even stronger, at 45 percent, with a peak in quick conceptions in late November. Meanwhile, the relationship between season and fecundability turned out to be about the same in Denmark and in Northern states and Canada.
The study used data on 14,331 pregnancy-planning women who had been trying to conceive for no more than six months, including 5,827 US and Canadian participants in the SPH-based Pregnancy Study Online (PRESTO) and 8,504 Danish participants in the Snart Gravid and Snart Foraeldre studies based at Aarhus University in Denmark. These studies follow women with detailed surveys every two months until they either conceive or have tried to conceive for 12 menstrual cycles, gathering data on everything from intercourse frequency and menstruation, to smoking and diet, to education and income.
[...] "Although this study cannot identify the reasons for seasonal variation in fertility, we are interested in exploring several hypotheses on seasonally-varying factors and how they affect fertility, including meteorological variables such as temperature and humidity, vitamin D exposure, and environmental exposures such as air pollution," Wesselink says.
Amelia K Wesselink, et al. Seasonal patterns in fecundability in North America and Denmark: a preconception cohort study. Human Reproduction, 2020; DOI: 10.1093/humrep/dez265