The household production literature emphasizes that technical or biological processes condition input selection by households in their production activities, along with prices and income. Exogenous variations in health, to the extent that they are perceived by individuals (heterogeneity), lead to correlations between inputs and health outcomes that cannot be used to derive causal conclusions. Therefore, estimates of health technology must be obtained from a behavioral model in which health inputs are themselves choices. Consistent estimates are reported of the effect of endogenous inputs, such as medical care, smoking, and fertility, on birth weight and fetal growth in the presence of health heterogeneity.