Presents a dynamic economic model of fertility behavior, estimated with age/year specific U.S. time series data and designed to explain and predict both current fertility rates and expected family size. The estimated model indicates that couples, on average, time their births to avoid periods when female wage rates, hence the time cost of children, are expected to be high. The estimates also show that most of the observed effect of income and female wages on fertility rates is due to the indirect influence of these variables on couples' expectations concerning future income and wages. The analysis also develops a decomposition of current fertility rates into timing and completed fertility components. This decomposition allows examination of the extent to which current low fertility rates are consistent with couples' fertility expectations. The estimates indicate substantial consistency, pointing to continuing low fertility rates apart from temporary increases during economic slowdowns. 53 pp. Ref.