It’s been said that timing is everything. But does that also apply to getting pregnant?

Apparently so, according to recent research led by scientists at the Boston University School of Public Health (BUSPH).

The study, which involved more than 14,000 women in the US, Canada and Denmark who were trying to get pregnant, analyzed data that included details on intercourse frequency, menstruation, smoking, diet, income and education.

Described as a first-of-its-kind study, the researchers focused on fecundability, which is the women’s probability of becoming pregnant during a single menstrual cycle. The women were tracked with detailed surveys until they either conceived, initiated fertility treatment or had tried to conceive unsuccessful for 12 menstrual cycles.

“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,” explained Amelia Wesselink, PhD, a postdoctoral associate in epidemiology at BUSPH.

North Americans in the study were more likely than Danes to begin trying to conceive in the fall (perhaps so the birth would occur in the summer when their work schedules may be less demanding).

Results of the study, which was published in Human Reproduction, suggest that seasonal variability does play a role in pregnancy…

  • In the US and Canada, the probability of becoming pregnant in late fall and early winter was 16% higher than in the spring.
  • In Denmark, the probability of becoming pregnant in the fall was 8% higher than in the spring.
  • In all countries, the probability of becoming pregnant was lowest in the spring.

The researchers controlled for other factors that might play a role in fecundability, including frequency of sexual intercourse, frequency of menstruation, smoking, consumption of sugar-sweetened beverages and medication use. These factors did not appear to explain the seasonal variation in fertility.

Even though the researchers do not know why the season appears to affect fecundability, they have some hypotheses that they would like to explore. These include temperature, humidity and vitamin D exposure.

Source: The study “Seasonal Patterns in Fecundability in North America and Denmark: A Preconception Cohort Study,” led by researchers at Boston University School of Public Health and published in Human Reproduction.