Researchers in the increasingly influential field known as fetal origins—the study of how fetal conditions affect long-term health—have to be pretty creative. Running controlled experiments on pregnant women is, after all, verboten. Still, scientists have made exciting finds in recent years by cleverly drawing on data from dramatic, naturally occurring “experiments.” By seeing what happens to fetuses when an entire population is exposed to, say, fasting, or mourning, or smog—conditions that would be deemed unethical if they were devised in a lab—we are gaining insights into how life before birth shapes life long afterward.
For quite a while, scientists have known that maternal food deprivation is bad news for fetuses, correlated with everything from coronary disease later on to skewed sex ratios at birth. (Normally, 105 boys are born for every 100 girls. But during food shortages and other times of population-wide stress, relatively more girls are born, probably because male fetuses are more fragile than female ones, and more susceptible to being miscarried.) Recent studies have uncovered new examples of this effect. The Chinese famine of 1958–61 saw male births decline sharply . Even fasting takes a toll: When Ramadan occurred very early in pregnancy, Arab mothers in Michigan were 10 percent less likely to have a son. And Muslims in Iraq and Uganda were 20 percent more likely to be disabled as adults if their mothers were in early pregnancy during the holiday .