The postponement of motherhood has drawn much attention in recent research. Yet,it is the U shape pattern of the mean age of the mother at first birth that observed in the data in 1950-2005. This phenomena can not be fully explained by the explanatory variables of the delay of motherhood documented in the empirical and theoretical literature. This paper develops a theoretical model to jointly consider females’ decisions on motherhood timing, as well as human capital accumulation, working, and resources spent on child rearing. The calibration results successfully generate the U shape pattern of age at maternity along the growth path. Our explanations differ from the existing human capital story, by taking into account that child rearing affects mother’s decisions on human capital investment both before and after motherhood. The model predicts that the delay in motherhood timing will continue.