You may have seen the headlines: “Older mothers tend to live longer,” was how TIME put it. NPR chose a more careful but similar phrasing: “Older moms take heart: You may be more likely to live longer.” But the study didn’t show what you might expect.
(Disclaimer: I wrote one of the many news briefs on this, though not the ones linked above. That’s why I had occasion to read the paper so closely.)
First of all, the older moms weren’t that old when they had their last child (which is the date the researchers counted: age at the last child’s birth, regardless of when or whether earlier siblings may have been born). Women who are older when they have their last child are likely to be older at menopause, but those dates are often separated by a decade, and clearly there are a lot of different factors at play in their social lives, not just their biology, in determining whether they have a child at a later age.
The women in this study were born on the order of 70 to 100 years ago, so we’re looking at women making decisions about family planning from roughly the 1940s through the 1970s. That’s nearly a generation of difference between subjects in the same study, although the researchers say they were only looking at women over 70 because going any younger would be too drastic of a difference in social factors.
But the biggest thing that didn’t always get reported about this study was that the controls—the women who were judged as NOT living to extremly old ages—were also very old. The median age of the extremely old women was 100; in the group of deceased women used as controls, the median was 92. So if a woman finished having kids (for any reason) before the age of 33, she was more likely to end up in the group that “only” lived, on average, to 92.
As a study to tell us about the health of the average woman who has babies late in life, this stinks. But that’s not what the study was meant to do. It’s one of the analyses that comes out of the Long Life Family Study, which enrolls not just individuals, but families in which several siblings live to exceptional old ages. The idea is that if there are genes for super-longevity, these families would have them.
So it was within this population that the researchers looked for links to fertility. Some historical studies had shown that women who finished having children later in life were likely to live longer. Women from those times didn’t have the same contraception options that we (well, some of us) do today, so the factors involved may be more biological.
The researchers were considering some evolutionary hypotheses that would link fertility and longevity: perhaps women that are fertile longer are somehow better able to conserve energy in a way that would help them live longer. And they admit (for example, in quotes in the NPR article) that studying the women’s actual age of menopause would be more useful in the quest for genes that can confirm their hypothesis. That data, they say, isn’t available yet.