The authors use the Islamic holy month of Ramadan as a natural experiment in fasting and fetal health. In Michigan births 1989-2006, they find prenatal exposure to Ramadan among Arab mothers results in lower birthweight and reduced gestation length. Exposure to Ramadan in the first month of gestation is also associated with a sizable reduction in the number of male births. In Census data for Uganda, Iraq and the U.S., the authors find strong associations between in utero exposure to Ramadan and the likelihood of being disabled as an adult. Effects are particularly large for mental (or learning) disabilities. They also find significant effects on proxies for wealth, the sex composition of the adult population and more suggestive evidence of effects on schooling and earnings. They find no evidence that negative selection in conceptions during Ramadan accounts for their findings, suggesting that avoiding Ramadan exposure during pregnancy is costly or the long-term effects of fasting unknown.