The age pattern of first-birth rates among U.S. women: the bimodal 1990s.

Between 1990 and 2002, the age pattern of Type I first-birth rates (i.e., the hazard of a first birth) among U.S. women was bimodal. This pattern, driven by changing differential fertility patterns among racial and ethnic groups, reached its apex at the mid-1990s and had almost vanished by the decade's end. Research on first-birth timing has tended to focus on Type II first-birth rates and therefore has failed to identify this larger, bimodal pattern. This article presents the benefits of using Type I rates, documents the emergence of the bimodal pattern via two new measures of bimodality, and uses a decomposition analysis to discuss the pattern's causes.