Women who wait longer to have their first child may live longer than mothers who start families when they're young, a US study suggests.
Later pregnancies can carry an increased risk of miscarriages and birth defects, but the study found women who give birth for the first time after age 25 are 11 per cent more likely to survive into their 90s than women who become first-time mothers at younger ages.
Researchers said it's likely that later childbirths are a reflection of better health, rather than being a cause of greater longevity.
"Our findings do not suggest that women should delay childbearing, as the risk of complications increases for later childbearing," said lead study author Aladdin Shadyab of the University of California San Diego.
"It is possible that women in our study who survived a pregnancy at an older age were healthier overall and thus were more likely to live longer," Shadyab added by email. "Alternatively, it is possible that women who had their first child at an older age were of a higher social and economic status, and therefore were more likely to live longer."
Over the past four decades, the average age of first-time mothers has climbed dramatically in the US, aided by a six-fold surge in first-birth rates among women 35 and older, researchers note in the American Journal of Public Health.
To explore the relationship between childbirth timing and longevity, researchers examined data on about 20,000 women enrolled in the Women's Health Initiative study from 1993 to 1998.
Women were typically around 75 years old when they joined the study and they were followed for up to 21 years.
Overall, 54 per cent of the women in the study survived to 90 years old.
These survivors were slightly older when they had their first child than the women who didn't reach 90, the study found.
About 13 per cent of the women never had children. Another 10 per cent of the women had just one baby, while 62 per cent had two to four kids and 15 per cent had at least five children.
Among about 15,000 participants for whom there was information on their age at childbirth, 47 per cent were younger than 25 when they had their first baby and another 37 per cent were between 25 and 29.
Women who had two to four children were more likely to live longer than mothers of just one child - at least for white women. This wasn't the case for black women.
The study also found that women who lived to age 90 were more likely to be college graduates, married, have a higher income and less likely to be obese or have a history of chronic disease.
Because the study is observational, it doesn't prove how pregnancy timing might cause women to live a longer life, the authors note.
It's also possible that the women who joined this study in their mid-70s already had a lot going for them health-wise, said Steven Austad, a researcher at the University of Alabama at Birmingham who wasn't involved in the study.
"Living to 90 isn't that exceptional if you've already lived to 75," Austad said by email.
The link between later pregnancy and longevity in the study might be explained by many other health, social and economic factors, said Roksana Karim, a researcher at the Keck School of Medicine at the University of California, Los Angeles.
"Women should certainly not guide their decision-making related to pregnancy on these findings," Karim, who wasn't involved in the study, said by email.