We estimate the child penalty using detailed personnel records that enable decomposition into distinct pay components. Our analysis reveals that the penalty is initially driven by reductions in time-based pay following childbirth. However, job-rank-based pay becomes increasingly significant over time, emerging as the dominant factor by the 15-year mark. These effects are interconnected: reduced working hours lead to lower performance evaluations, which subsequently limit promotion opportunities. Our theoretical model demonstrates that current promotion practices, which reward extended hours at entry-level positions, can generate production ineffciency. This finding suggests that addressing promotion practices could simultaneously reduce gender inequality and improve talent allocation, making a compelling business case for organizational reform.