We study the earnings and career profiles of employees who experience the birth of the first child, as compared to their childless co-workers. Using a difference-in-differences approach and a unique 12-year panel of personnel records from a large French company, we find that the arrival of a child creates a persistent penalty in earnings for mothers. The gap in internal promotions, both at the extensive and intensive margin, accounts for the vast majority of the motherhood penalty within the firm. We believe that firm-level policies on child-related leaves, if not gender-neutral, can exacerbate the motherhood penalty.