When a woman is born may influence when she runs out of eggs.© PunchstockThe season in which a woman is born influences the age at which she will go through the menopause, suggests a survey of northern Italians.
The survey, which looked at nearly 3,000 post-menopausal women at three clinics, revealed that those born in March showed the earliest menopause, at an average age of 48.9 years. At the other end of the scale, those born in October remained fertile until an average age of 50.3, with many lasting beyond 55.
The difference may arise because spring babies tend to be born with a smaller stash of eggs in their ovaries, suggests Angelo Cagnacci of the Modena General Hospital, who led the study. They might therefore run out earlier in life, leading to an earlier menopause.
Cagnacci's team quizzed the women on the age at which they entered menopause, and a suite of other factors thought to influence menopause timing: the age at which they first became fertile, whether they ever smoked, their education level and their type of employment. After accounting for these differences, a clear seasonal pattern emerged, the team reports in the journal Human Reproduction1.
Bumper baby
Previous studies have shown other differences based on time of birth. Babies born in autumn tend to be heavier, for example, and to live longer.
What causes this remains a mystery. One obvious difference is that spring babies gestate largely during the winter, whereas their autumn counterparts spend summer in the womb. It may be that differences in temperature, length of day, the mother's diet or infections may influence the developing child, Cagnacci says. Still, in today's world, where many people live in controlled environments and most foods are available at all times of year, such seasonal differences should be small.
Whatever the seasonal mechanism, Cagnacci suggests that this affects the number of eggs with which a female child is born. This in turn should help dictate when those eggs run out. There is some evidence that a woman may be able to add to her stock of eggs during her lifetime (see 'Could we defeat the menopause?), but this is expected to make a minimal contribution to her overall number of eggs.
Weighty results
The survey results do show that birth month influences age of menopause, says Tom Kelsey, who studies human reproduction at the University of St Andrews, UK. But he is surprised by the result.
ADVERTISEMENT
"It's amazing. I would have said the mother's body mass index would be the main variable," Kelsey says. He points out that there might not have been much difference in weight between the mothers of the women in this study, because the subjects were born in the years following the Second World War, when general nutrition was poor. If studies were done elsewhere, the mother's diet and weight might play a larger role, he says.
"It's perfectly possible that this is a real, general effect," Kelsey concedes. "But maybe there's just something about northern Italy. If you did the study again in North America, say, maybe other factors would come in."
© 2019 Nature is part of Springer Nature. All Rights Reserved.
