The PrePARED consortium creates a novel resource for addressing preconception health by merging cohorts. We describe our data harmonization methods and results. Individual-level data from 12 prospective studies were pooled. The crosswalk-cataloging-harmonization procedure was used. The index pregnancy was defined as the first post-baseline pregnancy lasting more than 20 weeks. We assessed the heterogeneity across studies by comparing preconception characteristics in different types of studies. The pooled dataset included 114,762 women, and 25,531 (18%) reported at least one pregnancy lasting more than 20 weeks of gestation during the study period. The index pregnancies were delivered between 1976 and 2021 (median=2008), at the mean age of 29.7±4.6 years. Before the index pregnancy, 60% were nulligravid, 58% had a college or higher degree, and 37% were overweight or obese. Other harmonized variables included race/ethnicity, income, substance use, chronic conditions, and perinatal outcomes. Participants from pregnancy-planning studies had more education and were healthier. The prevalence of pre-existing medical conditions did not vary substantially based on whether studies relied on self-reported data. Harmonized data presents opportunities to study uncommon preconception risk factors and pregnancy-related events. This harmonization effort laid the groundwork for future analyses and additional data harmonization.