Time Of The Season: Study Finds Late Fall, Early Winter Is Best When Trying For Baby

BOSTON — Thinking about adding a new member to the family? Scientists at Boston University say that couples tend to achieve conception at a quicker rate during the late fall or early winter. That timeline is especially relevant for residents of southern U.S. states, according to this first-of-its-kind piece of research.

First, birthday rates according to month were calculated. In the United States, birthdays tend to peak in early September. However, in northern U.S. states, and Scandinavia for that matter, birthdays usually increase a bit earlier than that, reaching their peak in the summer. While it’s undeniable that a variety of factors play a role in the popularity of birth months (November births are often linked backed to Valentine’s celebrations nine months prior), the research team at BU say that the seasons themselves greatly influence a couple’s chances of conceiving.

All in all, couples in the United States and Denmark are most likely to start trying for a baby in September, but they actually have much better odds of conceiving a few months later (late November, early December). This appears to be extra true for couples living in 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 Dr. Amelia Wesselink, postdoctoral associate in epidemiology, in a media release. “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. Interestingly, the association was stronger among couples living at lower latitudes.”

“Fecundability” means the odds of conception within the duration of a single menstrual cycle.

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The researchers used data on 14,331 pregnancy-planning women who had been attempting to become pregnant for a maximum of six months. Of that sampling, 5,827 women were from either the U.S. or Canada, and 8,504 were Danish. All of those participants had filled out a series of surveys every two months until each had either successfully conceived, or been trying for a total of 12 menstrual cycles. The questionnaires asked about menstruation cycles, intercourse frequency, diet, smoking habits, income, and education level.

North American women were more likely to try for conception in the fall than Danes, but even after accounting for that, season affected North American women’s fecundability by 16%. In comparison, Danes saw an 8% seasonal boost in the fall and subsequent drop in pregnancy rates by the spring each year. In southern U.S. states, the influence of season was even stronger: these women saw a 45% increase in pregnancies in late November.

Even after the study’s authors accounted for factors such as seasonal intercourse rates, sugary beverage consumption, smoking, and medication use, the study’s findings largely remained the same.

“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 concludes.

The study is published in Human Reproduction.

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