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Pregnancy is a known stressor on the body. But a new study published on Friday in Cell Metabolism found that while pregnancy accelerates the body’s biological clock, much of that effect is reversed after delivery, especially in people who breastfeed.

The results add to growing evidence that molecular aging, a process by which our cells accrue damage and dysfunction over time, doesn’t progress in a linear fashion — nor only forward.


Previous research, including a 2023 study also published in Cell Metabolism, has also found that being pregnant — much like Covid-19 or other infections and stressors — is associated with faster molecular aging in mice and humans, and these effects can be reversed to a degree. But the recent findings push those insights further by showing that the aging reversal was strongest in people who breastfed or had a lower body mass index.

Researchers made the findings by analyzing blood samples collected from approximately 120 women at various stages during pregnancy. About 65 women provided samples during the postpartum period, too.

“In a period of approximately 20 weeks, when we collected the samples in early to late pregnancy, genetic biomarkers were increasing by around one to two years, suggesting an increase in biological aging,” said Kieran O’Donnell, the study’s senior author and a professor of medicine at the Yale School of Medicine.


But the accelerated aging didn’t continue after delivery. “What was very surprising to us was the pronounced reversal of biological age at three months postpartum,” said O’Donnell. Biological age decreased after participants gave birth, “such that individuals at three months postpartum were looking biologically younger than at late pregnancy,” he said.

Researchers also found that a higher BMI was associated with higher biological age through pregnancy and postpartum, with an increase of between 0.7 and 1.4 years among women with a BMI of 30 compared to those with a BMI of 23, depending on the biomarker. Conversely, breastfeeding accelerated the rate of recovery, and women who breastfed exclusively had an average biological age about one year less than those who used formula. Researchers don’t yet know whether that was due to the loss in body mass linked with breastfeeding or other reasons.

“It’s really gratifying as a scientist to see your results replicated independently,” said Jesse Poganik, an instructor in medicine at Brigham and Women’s Hospital and the lead author of a 2023 paper on biological aging and pregnancy. “[We] didn’t have these other nice covariates that have now been analyzed like BMI and breastfeeding.”

Researchers hope to conduct follow-up studies that include samples taken prior to pregnancy to compare the aging during pregnancy and recovery with pre-pregnancy molecular age. This could help scientists determine whether postpartum recovery completely reverses the aging that occurred during pregnancy.

O’Donnell says the research team also hopes to identify pregnancy-specific biomarkers that could give a more exact measure of molecular aging. “I think it’s an interesting idea,” said Vadim Gladyshev, a professor of medicine at Harvard Medical School who specializes in antioxidant biology and was not involved in the study.

Poganik thinks further research on molecular aging during pregnancy could lead to specific interventions that benefit pregnant people. “This is one way that the science of aging can impact health outcomes, not just to improve our understanding,” he said.

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