Pregnancy triggers a permanent growth spurt in female bodies that works entirely without the help of growth hormone.

Female mammals experience a permanent increase in lean body mass and bone length after they give birth for the first time. Growth hormone was always thought to be the only way a body could physically get larger once adulthood began. Testing showed that even when growth hormone is completely absent, the physical expansion of the skeleton and muscles still happens during pregnancy. This reveals a shadow growth system that only activates during reproduction to prepare the body for the physical demands of motherhood. Understanding this secret pathway could lead to new ways to repair bone density or muscle wasting in people who cannot produce their own growth hormones.