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NEW YORK — Pregnancy is often described as a time of glowing health, but new research suggests it might also speed up the aging process in women. A study published in the Proceedings of the National Academy of Sciences finds that having been pregnant was associated with accelerated biological aging in women in their early 20s.

The international research team, which included scientists from Columbia University’s Aging Center, used cutting-edge “epigenetic clocks” – tests that measure chemical tags on DNA that change with age – to compare the biological ages of over 800 women, some of whom had been pregnant and some who had not. The clocks are like molecular odometers, revealing wear-and-tear on the body that may not yet be visible on the outside.

Across the board, the clocks showed that women who had experienced pregnancy, no matter how many times, had aged faster biologically than their peers who had never been pregnant. Each pregnancy was linked to around 6 months of additional biological aging on average.

Why Biological Age Is Important

You might be wondering, what exactly is biological aging and how is it different from chronological age? While chronological age is simply the number of birthdays you’ve celebrated, biological age reflects the true wear-and-tear on your body’s cells and tissues. It’s like comparing the mileage on two cars – a shiny new sports car and a beat-up taxi could both be 10 years old, but the high-mileage taxi will likely have more creaks and leaks under the hood.

The biological clocks — or epigenetic clocks — used in this study are on the cutting-edge of aging research. By measuring specific chemical modifications to DNA called methylation, they provide a remarkably accurate estimate of a person’s biological age. The clocks can reveal accelerated aging before its effects become apparent in wrinkles, gray hairs, or even doctor-diagnosable diseases.

How are scientists measuring these clocks? Imagine your DNA as a recipe book for making you. The words in the recipes are the genes, spelling out the ingredients and instructions for building proteins. But there are also chemical “post-it notes” stuck to the pages, telling the cell when and where to read each recipe. These notes are called epigenetic marks, and they change over time as we age and our cells face different demands. These so-called clocks are ways of “reading” these epigenetic post-it notes to estimate a person’s biological age. They focus on specific spots in the DNA where these chemical tags tend to change in a predictable way as we get older.

So what might explain this link between pregnancy and accelerated aging? The authors draw on an evolutionary theory that says there’s a tradeoff between reproduction and longevity. The idea is that the body’s resources are limited – energy invested in growing and nurturing a baby may get diverted from the mother’s own maintenance and repair. Over time, all those missed tune-ups add up to more rapid aging.

What This Study Found

The researchers tapped into data from the Cebu Longitudinal Health and Nutrition Survey, which has been tracking the health of over 3,000 Filipino women and their children since 1983. For their analysis, they focused on 825 women who were 20 to 22 years old in 2005. Detailed health and lifestyle data, including the number of pregnancies each woman had experienced, were collected through in-person interviews. Blood samples were also taken to analyze epigenetic patterns.

The researchers used six different epigenetic clocks to calculate each woman’s biological age. Some clocks were calibrated to predict chronological age, while others were designed to predict mortality risk or physiological decline. This multi-clock approach provided a comprehensive picture of the women’s biological aging across various domains.

In their initial cross-sectional analysis, comparing all 825 women at a single timepoint, the researchers found that those who had experienced at least one pregnancy had epigenetic age estimates 0.14 to 0.28 standard deviations higher than women who had never been pregnant, depending on the clock used. This translates to around 4 to 14 months of accelerated biological aging. When the number of pregnancies was analyzed as a continuous variable, each additional pregnancy was associated with about 2 to 5 months of accelerated aging per clock. These effects were seen even after controlling for potentially confounding factors like socioeconomic status, smoking, and the women’s chronological age.

To strengthen their case for a causal effect of pregnancy on aging, the researchers also conducted a longitudinal analysis on a subset of 331 women who got pregnant between the initial assessment and a follow-up conducted 3 to 9 years later. This comparison, where each participant essentially served as her own control, helped to account for hard-to-measure genetic and environmental factors that might influence both fertility and aging.

The longitudinal results showed that women who experienced more pregnancies in the interim had greater epigenetic aging according to two of the clocks used in the study — the Horvath and Hannum clocks — by around 0.06 standard deviations or 3 months per pregnancy. Statistically, this translated to explaining nearly 5% of the variation in epigenetic aging over the follow-up period.

Interestingly, the study found no similar aging effect in young men who had fathered children. The fact that accelerated aging was seen only in women suggests that pregnancy itself – as opposed to the general stresses of parenting – is what speeds the ticking of the biological clock.

Epigenetic Clocks Are Changing Perspectives Of Aging

Of course, this isn’t the first evidence that reproduction comes at a cost. From mayflies that perish after a single breeding season to panda moms who don’t eat for weeks while nursing, studies across the tree of life have shown that prolific parenthood often means a shorter lifespan. Even in humans, historical and epidemiological data show higher mortality in women who have had many children.

But this study is unique in quantifying the aging cost of pregnancy in young women, using the latest biomarkers. It also strengthens the case that there’s a causal link – in a subset of participants followed over time, women who experienced more pregnancies between the initial and follow-up measurements showed a greater acceleration of biological aging during that period.

Epigenetic clocks have revolutionized how we study biological aging across the lifecourse and open up new opportunities to study how and when long-term health costs of reproduction and other life events take hold,” said lead author Calen Ryan, Ph.D., an associate research scientist in Columbia’s Aging Center, in a university release. “Our findings suggest that pregnancy speeds up biological aging, and that these effects are apparent in young, high-fertility women. Our results are also the first to follow the same women through time, linking changes in each woman’s pregnancy number to changes in her biological age.”

The findings don’t mean that pregnancy is unhealthy or that women should avoid it for the sake of longevity. The apparent aging effects were subtle, and for most women the joys of motherhood will be well worth a bit of extra cellular mileage. But the study does suggest that the physical costs of reproduction start accumulating early and that supportive care for expectant and new mothers is important.

The authors acknowledge some limitations of their study. The epigenetic clock measurements only spanned a period of young adulthood, so it’s unclear how the aging trajectories might play out over a lifetime. The sample also came from a single population in the Philippines, where fertility patterns and health challenges may be quite different from other contexts.

Further research is needed to understand how the aging effects of pregnancy vary across contexts and later in life. But this study provides compelling evidence that for humans, as for many other species, reproduction carries costs that can be directly quantified in a mother’s cells. It underscores the incredible biological investment that mothers make to carry on the human lineage.

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