From the New York Times:

Good News for the Older Mothers

The Checkup

By PERRI KLASS, M.D. APRIL 3, 2017

The first time I got pregnant, I was a comparatively young mother, for my demographic: I was 25, in medical school, surrounded by classmates who, for the most part, were not reproducing yet. By the third pregnancy, 11 years later, I was over 35, which classified me, in the obstetric terminology I had learned in medical school, as an “elderly multigravida,” that is, someone who was having a child but not her first child, after 35. (If it was your first child, you were an “elderly primigravida,” or “elderly primip” for short — even as a medical student, I had a strong sense that no woman had invented this terminology.) …

National Vital Statistics Reports data released in January showed that in the United States, birthrates shifted in 2015: The birthrate for teenagers dropped to 22.3 births per 1,000 females ages 15 to 19 that year, a record low for the nation. And for women 30 through 44, the birthrates were the highest they have been since the baby boom era of the 1960s.

And as birthrates shift toward somewhat older mothers, researchers continue to look at what that says, both about who is getting pregnant when, and how that is associated with how their children do, especially when it comes to cognitive outcomes. …

The trend all over the developed world in recent years has been more women having more children later; mean age in the United States at birth of a first child increased from 24.9 to 26.3 from 2000 to 2014. And whether it’s a first child or a later child, more women giving birth are 35 and older, which is still classified as “advanced maternal age” (well, it beats “elderly”).

In a study published in February in the International Journal of Epidemiology, researchers looked at evidence from three different large longitudinal studies in Britain, from 1958, 1970 and 2000-2, each involving around 10,000 children. They were looking at the association between maternal age at children’s birth and children’s cognitive ability when tested at age 10-11.

In the two earlier studies, there was a negative association; maternal age 35-39 at birth was associated with poorer cognitive scores in the children, tested a decade later; the children who had been born to mothers 25-29 did better. On the other hand, for the most recent study, that association was reversed; the children born to the 35- to 39-year-olds did significantly better on the cognitive testing than the children born to the younger mothers.

What had changed over time? The researchers found that they could explain this reversal by correcting for the social and economic characteristics of the mothers; different women, in different circumstances, were having their children later in life.

Alice Goisis, a research fellow at the London School of Economics and Political Science and the Max Planck Institute for Demographic Research in Germany, and the lead author on the study, said, “the characteristics of older mothers have changed drastically over time.” In the older studies, she said, the women who were having children into their late 30s were more likely to be women who had many children, and possibly poorer, whereas in the later study, the millennium cohort study done in 2000-2, the older mothers were more likely to be educated, and socioeconomically better off. Twenty-six percent were giving birth to their first child at ages 35-39, as opposed to 11 percent in the 1958 study.

“One question I am often asked is whether these results are suggesting that women should wait to have children so they will have smarter children, and the answer is that our results are not addressing that,” Dr. Goisis said. “These women tend to be advantaged,” she said, and to take better care of themselves during pregnancy; they were less likely to smoke and more likely to breast-feed, compared to the younger mothers.

“Nowadays children of older mothers have, on average, better outcomes because of the characteristics of women who tend to have children at older ages,” Dr. Goisis said.