The influence of fertility on female labor supply has been studied extensively in Western societies, but little evidence is available in Sub-Saharan Africa. This article studies the impact of fertility on female time allocation to income-generating activities in Northern Tanzania. Because of the importance of extended households in African societies, we control for household composition and the position of women in their household. Possible endogeneity issues are addressed by estimating a first difference panel data model with instrumental variables. We find that the influence of fertility depends on the position of women in their household and the corresponding life cycle. Fertility has a negative effect for women who recently started their own household, but has a positive effect for women who had split off long before. We attribute this to the close relation between women’s position in the household and their social status. With increased status women obtain more bargaining power, hence more possibilities to spend time on income-generating activities. Consequently, over time the income effect becomes stronger than the substitution effect, leading to a positive effect of fertility on labor supply. We also find that women’s time spent on income-generating activities may be influenced by the number of older biological children, male adults, elderly and mother substitutes in the household.