Women who have children later in life are often warned about the risks, like pregnancy complications. But now there’s some good news for those who wait: According to a new study, moms who have their last child at age 35 or older may have better memories as they age compared with women who get pregnant when they are younger.

For women who don't want to have kids right away, or can't, this is the second bit of encouraging news in less than a week. A University of California San Diego study recently found that “older” moms (over 25 when they first gave birth) were 11% more likely to live to age 90, as well.

It's not clear why having a baby later in life would help your memory, but those women may experience a later-in-life surge of estrogen and progesterone, says lead author Roksana Karim, MD, assistant professor of clinical preventive medicine at the University of Southern California’s Keck School of Medicine. In animal studies, these hormones have been shown to benefit brain function and growth.

Scientists also know that, in humans, estrogen and progesterone decline after menopause. And often, so do cognitive skills like memory. Having higher levels of these hormones at an older age may have a protective effect, Dr. Karim and her colleagues hypothesized.

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In the study, the researchers gave 830 post-menopausal women a series of tests to measure different types of memory, attention and concentration, planning, visual perception, and reaction time. They also recorded their reproductive history, including when they began menstruating, got pregnant, and used birth control. (The researchers did not ask about fertility treatments, so it's unknown whether any of the women used them.)

After taking into account other factors like age, race, ethnicity, income, and education, the researchers determined that women who had their last pregnancy after age 35 scored highest on tests of verbal memory (which involved repeating a list of words or retelling a story after being distracted). Women who were at least 24 during their first pregnancy also had better executive function—a measure of attention control, working memory, reasoning, and problem-solving—compared to those who got pregnant earlier. 

This is the first study to investigate the link between last-pregnancy age and cognitive function in later life, says Dr. Karim. "Based on the findings, we would certainly not recommend that women wait until they're 35 to close their family,” she said in a press release. “But the study provides strong evidence that there is a positive association between later age at last pregnancy and late-life cognition.”  

The research, which was published in the Journal of the American Geriatrics Society, revealed other patterns, as well. The study also found that women who used hormonal birth control for more than 10 years, or who started their menstrual cycle before turning 13, also had better mental power after menopause.

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"Starting your period early means you have higher levels of the female sex hormone being produced by the ovaries," Dr. Karim said. "Girls are receiving the optimal levels early, so it's possible that their brain structures are better developed compared to those who are exposed to estrogen levels associated with menstrual cycles at a later age."

And birth control pills or other hormonal contraception "maintain and sustain a stable level of sex hormones in our blood stream," Dr. Karim said. "Stable is good."

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Some research has suggested that women’s memory and thinking skills can suffer while they’re expecting—a phenomenon known as “pregnancy brain,” said senior author Wendy Mack, PhD, professor of preventive medicine at the Keck School of Medicine. But she thinks these studies did not follow women for enough time afterward to determine long-term effects. 

“The many bodily changes and psycho-social stressors during pregnancy also can impact women's cognitive and emotional functions," she said, so it could be difficult to detect hormone-related benefits right away.

The observational study was only able to find an association between these reproductive factors and mental abilities—not a cause-and-effect relationship. And there are still plenty of reasons to not delay if you do want children: The older a pregnant woman is, the more risks she and her baby face, especially over age 40.