Based on findings from high-income countries, typically economists hypothesize that having more children unambiguously decreases the time mothers spend in the labor market. Few studies on lower-income countries, in which low household wealth, informal child care, and informal employment opportunities prevail, find mixed results. Using Mexican census data, I find a positive effect of an instrument-induced increase in fertility on maternal employment driven by an increase in informal work. The presence of grandparents and low wealth appear to be important. Econometric approaches that allow extrapolating from this complier-specific effect indicate that the response in informal employment is non-negative for the entire sample.