Having a baby physically shrinks part of a woman's brain. Having a second baby shrinks a totally different part. Scientists in Amsterdam just figured out why, and the explanation involves the same process that happens in teenage brains. This is from a research group in Amsterdam called the Pregnancy Brain Lab. They published their findings in Nature Communications on February 19, 2026. The team scanned the brains of 110 women. 40 were about to have their first baby, 30 were about to have their second, and 40 had never been pregnant. They scanned everyone before pregnancy and again after birth. The results were so consistent that a computer program could look at any of those brain scans and correctly tell whether the woman had been pregnant. Every single time. When a woman has her first baby, the biggest changes happen in the part of the brain that handles thinking about yourself and other people. The same region that runs daydreaming and inner monologue. That whole area visibly shrinks. And it stays shrunk for at least six years after birth, according to a 2021 follow-up study by the same team. When she has a second baby, that same area shifts a little more, but the biggest changes happen somewhere else. They happen in the part of the brain that controls what you focus on, and the part that controls how your body moves. Even the wiring between the brain and the muscles becomes more efficient. Lead researcher Milou Straathof said it looks like the brain rewiring itself for taking care of more than one kid at a time. The shrinking sounds bad. The lab compares it to what happens in teenage brains during puberty. Hormones flood the brain and trigger a kind of cleanup. Weak connections between brain cells get cleared away. The strong ones stay and get stronger. The brain ends up smaller, but the connections that remain work faster. The hormonal flood of pregnancy seems to do the same thing. Elseline Hoekzema, who runs the Pregnancy Brain Lab and has been studying this since 2017, told CNN: sometimes less is more. The pattern is layered. The first pregnancy does the deep work on identity and how a mom thinks about her baby. The second pregnancy adds a new layer focused on attention and movement. About one in five new mothers globally develops postpartum depression. The same brain circuits being remodeled here are the ones tied to mood and bonding with the baby. Mapping what a healthy maternal brain looks like is the first step toward catching when something goes wrong.