Women who become pregnant later in life are often warned about the health risks, but there may be an upside to becoming a mom when you're older: a new study suggests that women who have their last baby after age 35 are mentally sharper later.

The new findings may be “good news for professional women who plan to have a family later in life,” said the study’s lead author, Dr. Roksana Karim, an assistant professor for clinical preventive medicine at the Keck School of Medicine at the University of Southern California.

The study comes at a time when many women in the U.S. are delaying childbirth —there's practically a baby boom among women age 35 and older.

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To explore the effects of late life pregnancy on brain health, Karim and her colleagues re-analyzed data gathered in two clinical trials, which were designed to test the impact of soy supplements and estrogen in older women. Those trials collected information on reproductive history and also evaluated cognition in 830 women, ages 41 to 92, according to the report published in the Journal of the American Geriatrics Society.

It wasn’t just late-life pregnancy that appeared to be beneficial to women’s brains. The researchers also found that being on birth control pills for more than 10 years was associated with better verbal memory and the ability to think critically in old age. The reason may be that oral contraceptives maintain a stable level of sex hormones, Karim said.

The study is an association and doesn't prove direct cause-and-effect, so women shouldn't deliberately delay pregnancy thinking that it'll protect their cognition. One reason: fertility drops significantly after age 35. But with so many warnings about the dangers of delaying pregnancy, "this is one finding that is in the positive direction," said Dr. Hygriv Simhan, a professor at the University of Pittsburgh Medical Center and chief of maternal-fetal medicine at UPMC’s Magee-Women’s Hospital.

It's possible that some unknown factor might explain the findings — women who are reproductively healthy enough to have children later in life may have brains that resist dementia or memory problems.

Although the researchers tried to account for issues such as disparities in income and education, there might be something else about the women that could explain the associations found by the study, Karim allowed.

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Dr. Whitney You suspects that there may be some hidden biases in the data due to the fact that women who opt to delay pregnancy often have a higher educational level and socio-economic status, compared to women who don’t.

“They did ‘correct’ for this, but you can only correct so much,” said You, an assistant professor of obstetrics and gynecology at Northwestern University’s Feinberg School of Medicine. “We don’t know whether that is driving this result or whether it’s truly physiologic.”