Pregnancy is linked to faster epigenetic aging in young women. A study of 1,735 young adults in the Philippines found that women who had pregnancies showed faster biological aging, as measured by epigenetic clocks, compared to those who had not. Cross-sectionally, the number of pregnancies was associated with accelerated epigenetic aging across all six epigenetic clocks. Longitudinally, increases in the number of pregnancies were linked to accelerated aging in two of the clocks. In contrast, the number of pregnancies a man reported fathering was not associated with epigenetic aging in same-aged men. These effects were robust to socioecological, environmental, and immunological factors, supporting the hypothesis that pregnancy accelerates biological aging and that these effects can be detected in young women in a high-fertility context. The study used six epigenetic clocks to measure biological aging and found that pregnancy was associated with faster aging, particularly in young women. The findings suggest that pregnancy accelerates biological aging, especially when carried out early in women's reproductive careers, and that these effects may be detectable starting at a relatively young age. The study also highlights the importance of longitudinal approaches in studying the relationship between reproduction and aging, as they minimize the cumulative confounding effects of social and environmental factors. The results support the evolutionary theory that reproduction comes at the expense of somatic maintenance and repair, leading to accelerated biological aging. The study provides evidence that pregnancy accelerates biological aging in a healthy, young adult population.Pregnancy is linked to faster epigenetic aging in young women. A study of 1,735 young adults in the Philippines found that women who had pregnancies showed faster biological aging, as measured by epigenetic clocks, compared to those who had not. Cross-sectionally, the number of pregnancies was associated with accelerated epigenetic aging across all six epigenetic clocks. Longitudinally, increases in the number of pregnancies were linked to accelerated aging in two of the clocks. In contrast, the number of pregnancies a man reported fathering was not associated with epigenetic aging in same-aged men. These effects were robust to socioecological, environmental, and immunological factors, supporting the hypothesis that pregnancy accelerates biological aging and that these effects can be detected in young women in a high-fertility context. The study used six epigenetic clocks to measure biological aging and found that pregnancy was associated with faster aging, particularly in young women. The findings suggest that pregnancy accelerates biological aging, especially when carried out early in women's reproductive careers, and that these effects may be detectable starting at a relatively young age. The study also highlights the importance of longitudinal approaches in studying the relationship between reproduction and aging, as they minimize the cumulative confounding effects of social and environmental factors. The results support the evolutionary theory that reproduction comes at the expense of somatic maintenance and repair, leading to accelerated biological aging. The study provides evidence that pregnancy accelerates biological aging in a healthy, young adult population.