Abstract: To estimate the causal impact of fertility on women’s labor force participation, we use the occurrence of multiple first births and the sex of the first births as measures of exogenous shocks to fertility. Since twins at first birth occur relatively rarely in one country, we use all available Demographic and Health Surveys. We find that preference for children of multiple sexes and sons are predominant factors in determining how women adjust their fertility decisions to the outcome of their first two births. Decisions regarding fertility and labor vary considerably depending on a woman’s age and geographic location. Variation in infant mortality also plays a role, as effects are often quite different on total fertility from the effects on the number of surviving children. Throughout the developing world, women have more children if they had twins in the first birth or if the first two births were the same sex. Son preference predominates in South Asia, where women have fewer children if either or both of their first two children born were sons. While many women in developing countries are similar to women in the U.S. in that they are less likely to participate in the labor force as a result of having more children, in a number of cases the opposite seems to be the case. We provide a number of possible explanations for why we might see such differences.