Pregnancy lab test dynamics resemble rejuvenation of some organs and aging of others

Pregnancy and aging are associated with stress on the body and show multi-system physiological changes. We asked whether we can learn about aging from the changes in pregnancy. To do so, we analyzed weekly cross-sectional data on 70 lab tests from 300,000 pregnancies and 1.4 million non-pregnant females aged 20-89. Using a biological age model trained on non-pregnant females, we observed that pregnant females' apparent age dropped by 5 years in the first trimester, rose by 20 years toward delivery, and recovered postpartum. Pregnancy complications increased apparent age by 2-6 years. Certain systems exhibited apparent rejuvenation - opposite trends in pregnancy vs aging - including renal, iron, and most liver tests. Others, such as coagulation, thyroid, muscle, and metabolism, showed apparent aging. Notably, the aging-like mechanisms of pregnancy differed from normal aging, suggesting superficial similarity, whereas the rejuvenation-like mechanisms may offer clues for slowing aspects of biological aging.