Have you ever wondered why Mamá looked so old when she was only 24 when she was pregnant? Or why your best friend seemed overly exhausted during her sixth month of pregnancy? It might look obvious—being a mom and/or pregnant is an around-the-clock job, after all. However, new research found that the blood cells of pregnant women, in fact, age at an exaggerated speed.

The new studies published this month in Proceedings of the National Academy of Sciences found pregnancy “has a big impact on a woman’s body and biological age.” 

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According to Calen P. Ryan, an associate research scientist at the Columbia Aging Center in New York who led the new research, the study found that the biological age of young women who had been or were pregnant tended to be higher than that of women who hadn’t conceived.

But how did they analyze biological age?

As the Washington Post explained, scientists used what they call “biological-age clocks.” These AI-powered algorithms “examine the patterns of specialized chemical markers found on the outside of some genes.” The markers “accumulate and change in response to our age, health, and lifestyles.” This is what they know as epigenetics.

The algorithms use the markers to estimate the relative age of cells. The interesting part is that this measure, often called biological age, can differ from someone’s chronological age.

This means that even if you’re 24, your cells can be, in fact, older.

Exactly why do pregnant women age faster?

The study Columbia University conducted involved DNA samples from 825 young women. The scientists found that “each individual pregnancy a woman reported was linked with an additional two to three months of biological aging,” The Guardian reported. On the other hand, women who had been pregnant more often during a six-year follow-up period showed a greater increase in biological aging during that period.”

Furthermore, the researchers found that this aging happened despite socioeconomic status, smoking, genetic variations, or environmental differences.

Ironically, they didn’t find biological aging in 910 same-aged men who had fathered pregnancies.

“Our findings suggest that pregnancy speeds up biological aging and that these effects are apparent in young, high-fertility women,” Ryan told The Guardian. “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.”

“Many of the reported pregnancies in our baseline measure occurred during late adolescence when women are still growing,” he continued. “We expect this kind of pregnancy to be particularly challenging for a growing mother, especially if her access to healthcare, resources, or other forms of support is limited.”

The added component of stress

Another study published in March in Cell Metabolism also found that “pregnancy seemed to be aging the incipient moms,” especially as they approached full term. According to researchers, “their blood cells’ DNA appears to be as much as two years older than it had been earlier in the pregnancy.”

And it might have a lot to do with the stress during pregnancy, the Washington Post reported.

However, Kieran J. O’Donnell, an assistant professor at the Yale Child Study Center who oversaw one of the studies, shared some encouraging news.

This aging “seemed to reverse for most women within three months after birth.” In fact, their patterns of DNA markers “soon reverted to an earlier, more youthful state.” And for some new moms who’d breastfed exclusively in the first three months postpartum, the DNA markers “overshot the mark, leaving them apparently younger biologically than before, by as much as eight years.”

In short, it seems like our biological clocks travel in time, back and forth, during pregnancy.