Objective: We sought to prospectively examine whether childbearing is associated with higher incidence of the metabolic syndrome (MetS) after delivery among women of reproductive age. Study Design: In 1451 nulliparas who were aged 18-30 years and free of the MetS at baseline (1985-1986) and reexamined up to 4 times during 20 years, we ascertained incident MetS defined by the National Cholesterol Education Program Adult Treatment Panel III criteria among time-dependent interim birth groups by gestational diabetes mellitus (GDM): (0 [referent], 1 non-GDM, 2+ non-GDM, 1+ GDM births). Complementary log-log models estimated relative hazards of the MetS among birth groups adjusted for race, age, and baseline and follow-up covariates. Results: We identified 259 incident MetS cases in 25,246 person-years (10.3/1000 person-years). Compared with 0 births, adjusted relative hazards (95% confidence interval [CI]) were 1.33 (95% CI, 0.93-1.90) for 1 non-GDM, 1.62 (95% CI, 1.16-2.26) for 2+ non-GDM (P trend = .02), and 2.43 (95% CI, 1.53-3.86) for 1+ GDM births. Conclusion: Increasing parity is associated with future development of the MetS independent of prior obesity and pregnancy-related weight gain. Risk varies by GDM status. © 2009 Mosby, Inc. All rights reserved.