Babies are most likely to be conceived in late autumn and least likely to be conceived in spring, according to a large study in North America and Denmark.
Births tend to spike at certain times of the year, but this may reflect seasonal variation in when women try to get pregnant rather than when they are most likely to conceive, says Amelia Wesselink at Boston University.
To find out when couples have the best chance of conceiving, Wesselink and her colleagues studied almost 6000 women in the US and Canada and more than 8500 women in Denmark who were trying to get pregnant without using fertility treatment. They were surveyed every eight weeks to find out if they had conceived and answered a questionnaire about their lifestyle.
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The researchers first looked at when the women started trying to get pregnant. They found that early autumn, particularly September, was the most popular time. This may reflect a preference for a baby to be born earlier in the summer, when it may be easier to take time off work, or to avoid being pregnant during the hot later summer months, says Wesselink.
The researchers then looked at when women were most likely to actually become pregnant. They adjusted the results to account for seasonal variations in pregnancy attempts and other factors that may differ across the year, such as how frequently couples had sex, their body mass index, physical activity and stress levels.
The chance of conceiving in a given menstrual cycle was 16 per cent higher for North American women and 8 per cent higher for Danish women in late November and early December when compared with late May and early June, when their chances of conceiving were lowest.
Why fertility seems to vary in this way is still a mystery, says Wesselink. She and her colleagues now plan to investigate whether seasonal fluctuations in temperature, humidity or air pollution explain changes in women鈥檚 fertility across the year. They are also interested in whether women鈥檚 fertility follows similar seasonal trends in the southern hemisphere.
Human Reproduction
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