I provide new evidence about the negative impact of family size on mothers' participation to the labor market for the particular region of Reunion Island and mainland France. Based on an instrumental variables (IV) model of labor supply, exploiting two exogenous increases of fertility through sex mix composition and multiple births, I find that the negative causal impacts are quite different. For Reunionese mothers the presence of a third child decreases the probability to participate of about 15 percentage points, while, the same effect is two times less important for French mothers. This finding reinforces the idea that family policies should use specific tools to facilitate the return to the labor force of Reunionese mothers.