In this study we assume that fertility decisions are made one birth at a time and use longitudinal data collected from 401 married couples over a two-year period to explore how having a child affects two types of fertility motivation and three types of fertility desires. Using a series of five constrained multiple regression analyses, we tested the effects of two childbearing variables on these five types of motivation and desires in the context of a large set of control variables also hypothesized to affect fertility motivation and desires. The results demonstrate that the childbearing variables have a substantial effect in all five regression models. Specific findings indicate that childbearing stimulates greater positive motivation for childbearing and an increase in the number of children desired. Although this situation would appear to create a positive feedback loop in which each child born further increases the motivation and desire for children, the findings also suggest three different mechanisms whereby childbearing causes a counterbalancing regulation of that loop. These mechanisms include a negative motivation mechanism, a satiation mechanism, and a delay mechanism. The effect of these mechanisms on the termination of childbearing is considered in conjunction with a fourth mechanism, the achievement of desired family size.