The gender imbalance in science, technology, engineering, and math (STEM) fields has remained constant for decades and increases the farther up the STEM career pipeline one looks. Why does the underrepresentation of women endure? This study investigated the role of parenthood as a mechanism of gender-differentiated attrition from STEM employment. Using a nationally representative 8-year longitudinal sample of US STEM professionals, we examined the career trajectories of new parents after the birth or adoption of their first child. We found substantial attrition of new mothers: 43% of women leave full-time STEM employment after their first child. New mothers are more likely than new fathers to leave STEM, to switch to part-time work, and to exit the labor force. These gender differences hold irrespective of variation by discipline, race, and other demographic factors. However, parenthood is not just a “mother’s problem”; 23% of new fathers also leave STEM after their first child. Suggesting the difficulty of combining STEM work with caregiving responsibilities generally, new parents are more likely to leave full-time STEM jobs than otherwise similar childless peers and even new parents who remain employed full time are more likely than their childless peers to exit STEM for work elsewhere. These results have implications for policymakers and STEM workforce scholars; whereas parenthood is an important mechanism of women’s attrition, both women and men leave at surprisingly high rates after having children. Given that most people become parents during their working lives, STEM fields must do more to retain professionals with children.