Young women leaving school have to make important choices about family and employment. But a lack of opportunities may severely limit the possibility of achieving their life-long potential. We here report from a large-scale  cluster-randomized trial seeking to empower young women in Tanzania on health and entrepreneurship. The entrepreneurship arm leads to a remarkable increase in self-employment and income, but also to an increase in fertility and teen pregnancies. We show that the positive effect on income can be explained by changes in preferences and knowledge and an occupational shift. The increase in fertility is then explained by an income effect, where higher incomes makes it affordable to raise a child earlier. Our results carry an important message to policy makers about the potential, but also the complexity, of female empowerment in low-income contexts.