Where have all the young men gone? Using sex ratios to measure fetal death rates.

Fetal health is an important consideration in policy formation. Unfortunately, a complete census of fetal deaths, an important measure of overall fetal health, is infeasible, and available data are selectively observed. We consider this issue in the context of the Clean Air Act Amendments of 1970 (CAAA), one of the largest and most influential environmental regulations in the history of the United States. We discuss a model of potential bias in measuring observed fetal deaths, and present the sex ratio of live births as an alternative fetal health endpoint, taking advantage of the finding that males are more vulnerable to side effects of maternal stress in utero. We find the CAAA caused substantial improvements in fetal health, in addition to previously identified reductions in post-natal mortality.