In this paper, I study the relationship between fertility and old-age support in developing countries. I present a two-period model in which parents decide on the quantity and the quality of children and savings during childbearing years and children choose transfers to parents during parents’ retirement years. Using this framework, I show that an exogenous increase in parents’ fertility has an ambiguous effect on old-age transfers. On the one hand, total transfers increase due to the rise in the number of children, although each child transfers less as the number of siblings rises. On the other hand, the increase in fertility changes parents’ optimal choices of human capital and savings. In particular, I show that the quantity-quality trade-off is weaker when old-age transfers take place. In the empirical analysis, I use individual-level data from China and Indonesia to test the model predictions. I exploit twinning in the first birth as a source of exogenous variation in the number of children. I find that twinning increases completed fertility of older mothers. Then, I show causal evidence that elder parents with more children receive more old-age transfers. Lower parental savings do not explain the latter effect. I find that children with more siblings make smaller transfers to parents. Parents respond to the increase in fertility by investing less in children’s education. Despite children having lower human capital, the estimates indicate that parents with more children consume more during old age.