In this paper, we investigate gender differences in workers' career development within and outside the firm to explain the existence of gender wage gaps. Using Danish employer-employee matched data, we find that good female workers are more likely to move to better firms than men but are less likely to be promoted. Furthermore, these differences in career advancement widen after the first child is born. Our findings suggest that career impediments in certain firms cause the most productive female workers to seek better jobs in firms where there is less gender bias.