Women in their early 20s who have been pregnant are “biologically older” than those who have never been pregnant, and by some measures, this age gap seems to widen in people who have had multiple pregnancies, a new study suggests.

The research, conducted in the Philippines, used various tools to look at people’s epigenetics, meaning the chemical tags attached to their DNA. These tags don’t change the DNA’s underlying code but rather help control which genes are activated and to what degree. The new study specifically looked at methyl groups, a type of molecule long linked to different aspects of the aging process.

By studying patterns of methylation seen throughout the human life span, scientists have created a number of “epigenetic clocks” that can be used to assess a person’s biological age. While chronological age simply reflects how long someone’s been alive, biological age reflects their physiological state and chances of age-related diseases and death.

“What epigenetic clocks are doing is they’re serving a predictive function rather than a sort of causal explanation,” said first study author Calen Ryan, an associate research scientist in the Columbia Aging Center. “They’re trained to predict things that we think of as representing aspects of aging.” So one clock may be designed to predict a person’s chronological age, while others predict a person’s likelihood of death and still others estimate the length of their telomeres, the protective caps at the end of DNA that keep it from fraying.

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The research, published Monday (April 8) in the journal PNAS, used six different epigenetic clocks to make predictions about 1,735 young women and men in the Philippines. The full group had blood samples taken in 2005, between the ages of 20 and 22. A subset of the women — around 330 — who became pregnant in the years following their first blood sample also had a second sample taken about four to nine years afterward.

Across all of the clocks used, women who’d had at least one pregnancy showed accelerated aging compared with women with no pregnancy history, the analysis revealed; the pregnancies included those that resulted in miscarriages, stillbirths and live births. The pattern still showed up when the scientists controlled for other factors that also affect a person’s rate of biological aging, such as socioeconomic status, smoking history and some genetic risk factors.

The researchers also found that women who’d had more pregnancies showed faster aging than those with fewer pregnancies “for all six of the clocks,” Ryan told Live Science. “We do not find that relationship among the men we looked at cross-sectionally.” In other words, the number of pregnancies a man fathered didn’t seem to affect the speed at which his epigenetic clock ticked.

(Notably, the men looked biologically older than the women overall, regardless of pregnancy status; it’s just that impregnanting people didn’t increase the men’s biological ages even higher. This pattern of biological aging in men is consistently seen across epigenetic-clock studies and may be connected to men generally dying at younger ages than women, Ryan said.)

The team then looked at the 330 women they followed over time, to see if there were differences between the women’s first and second blood samples. In that analysis, experiencing more pregnancies also was associated with faster aging compared with fewer pregnancies. However, this pattern showed up for only two of the six clocks — specifically the two designed to predict chronological age.

Based on all of these data, the team estimates that each pregnancy was tied to about 4 to 4.5 months of biological aging among the women in the study.

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The study’s findings may have been affected by where it was conducted. For instance, people’s access to adequate nutrition, health care and social support during pregnancy vary throughout the Philippines, and these factors may influence the extent to which pregnancy influences aging. It’s also relevant that most epigenetic clocks have been confirmed to work well at tracking aging in white people in developed countries, but many clocks still need to be fully validated in people of other demographics elsewhere in the world, Ryan noted.

“They’re still basically our best measures yet,” but they could likely be improved for different populations, he said. More work is also needed to tease out the effects of parenting on aging from those tied to being pregnant and giving birth, the authors noted in their report.

In addition, “these women are quite young at the time of the sample,” Ryan said of the study participants. So it’s not clear if women who are older at the time of their first pregnancy would show the same patterns. That said, it was helpful for the team to study young women because the researchers were trying to see if biological aging tied to pregnancy could be seen early, before the health outcomes of accelerated age show up.

If you can catch this accelerated aging early, that could theoretically inform future treatments to help prevent or reverse the process, Ryan said — although at this early stage of research, it’s unclear what such treatments would entail.

Similar upticks in biological aging have been seen in some other contexts, but not all. For example, they’ve been observed among Filipino women in the U.S. but not in women in Finland. A recent Yale study also found that epigenetic clocks accelerate during pregnancy but that much of that effect disappears after the child’s birth, especially in people who breastfeed.

So “we do have decent evidence of biological aging being sped up from pregnancy, but maybe not in all contexts,” Ryan said.

For now, this new study is helping scientists start to unpack the impact of pregnancy on the aging process. Someday, though, it could pave the way for medical interventions.

“My hope is that we can start to maybe use tools like this [epigenetic clocks] to identify at-risk individuals,” meaning people who may age more with each pregnancy, Ryan said. If they can identify factors that help buffer against biological aging, scientists could potentially design interventions that mimic those factors in people more susceptible to it.

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