In developing countries, employment rates for mothers with young children are relatively low. This study analyzes how maternal labor market outcomes in Argentina are affected by the preschool attendance of their children. Using pooled household surveys, we show that 4-year-olds with birthdays on June 30 have sharply higher probabilities of preschool attendance than children born on July 1, given enrollment-age rules. Regression-discontinuity estimates using this variation suggest that preschool attendance of the youngest child in the household increases the probability of full-time employment and weekly hours of maternal employment. We find no effect of preschool attendance on maternal labor outcomes for children who are not the youngest in the household.