Second demographic transition (SDT) theory posits that increased individualism and secularization have contributed to low fertility in Europe, but very little work has directly tested the salience of SDT theory to fertility trends in the U.S. Using longitudinal data from a nationally representative cohort of women who were followed throughout their reproductive years (NLSY79), this study examines the role of several key indicators of the second demographic transition (secularization, egalitarianism, religious affiliation, and female participation in the labor market) on fertility behavior over time (1982–2006). Analyses employ Poisson estimation, logistic regression, and cross-lagged structural equation models to observe unidirectional and bidirectional relationships over the reproductive life course. Findings lend support to the relevance of SDT theory in the U.S. but also provide evidence of "American bipolarity" which distinguishes the U.S. from the European case. Furthermore, analyses document the reciprocal nature of these relationships over time which has implications for how we understand these associations at the individual-level