Until the early decades of the 20th century, women spent more than 60% of their prime-age years
either pregnant or nursing. Since then, the introduction of infant formula reduced women's comparative
advantage in infant care, by providing an effective breast milk substitute. In addition, improved medical
knowledge and obstetric practices reduced the time cost associated with women's reproductive role.
We explore the hypothesis that these developments enabled married women to increase their participation
in the labor force, thus providing the incentive to invest in market skills, which in turn reduced their
earnings differential with respect to men. We document these changes and develop a quantitative model
that aims to capture their impact. Our results suggest that progress in medical technologies related
to motherhood was essential to generate a significant rise in the participation of married women between
1920 and 1950, in particular those with young children.