Having kids is costly. Not just financially, but biologically, too. At least in theory. Evolutionary biologists propose that bodies may have limited energy to split between reproduction and long-term maintenance.
Now a team of Finnish scientists has used decades of data from a massive twin study spanning 15,000 women to ask whether the arc of childbearing—how many children a woman has, and when she has them—could influence the rate at which she ages. Participants in the study were invited to complete a survey in 1975 and their lives were followed through 2020.
The researchers, from the University of Helsinki and the Minerva Foundation Institute for Medical Research, found a U-shaped pattern. Women who had two to three children and gave birth between the ages of 24 and 38 fared best. Women who had too few or too many fared the worst. The scientists controlled for smoking, alcohol use, weight, and education and published their results in Nature Communications.
“From an evolutionary biology perspective, organisms have limited resources such as time and energy,” explained doctoral researcher and co-author Mikaela Hukkanen, in a statement. “When a large amount of energy is invested in reproduction, it is taken away from bodily maintenance and repair mechanisms, which could reduce lifespan.”
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Having more than four kids, in particular, was associated with shorter lifespans than average, and more accelerated biological aging. Childless women also aged more rapidly than women with a few children, though the researchers suggested that this unexpected result might be explained by other lifestyle- or health-related factors that weren’t accounted for by the study: any underlying health issues that could have reduced fertility, different social support systems later in life, or different disease risks. For instance, certain kinds of pregnancies or lactation patterns can affect one’s cancer risk.
The researchers were quick to point out that the results don’t suggest certain childbirth age or childbearing numbers actually cause healthier aging, nor that women should shape their reproductive choices around the findings, particularly since the number of children women have today has fallen, and age of first birth has increased. (The twins in the study were born between 1880 and 1957.)
“An individual woman should therefore not consider changing her own plans or wishes regarding children based on these findings,” said study author Miina Ollikainen.
The results, which took biological aging markers from blood samples into account, support earlier findings based on mortality data. According to these epigenetic blood markers, women who had many children or no children were biologically slightly older than their chronological age. Women who had children at a very young age were also more likely to have an accelerated biological age.
“Our results show that life history choices leave a lasting biological imprint that can be measured long before old age,” added Miina Ollikainen.
In this twin cohort at least, the price tag of motherhood showed up in the extremes. The child-rearing middle ground seemed to be the safest bargain for the long game. 
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