The purpose of this longitudinal study was to investigate whether there are distinct etiological processes explaining dual usage of alcohol and conventional cigarettes by mothers from preconception through the early parenting years. Data on 8800 biological mothers were drawn from the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B), representative of US births in 2001. A general growth mixture model (GGMM) was used to empirically identify developmental trajectories of maternal smoking and drinking over the 5-6-year study period. Six classes defined by alcohol consumption and cigarette smoking were identified. These included a nonsmoking, low probability of drinking class (41%), and two drinking classes displaying no smoking with either moderate (26%) or escalating high (8%) probability drinking. Additionally, two predominantly smoking classes were identified, one displaying temporary reduction in smoking during pregnancy and low probability of drinking (11%) and one following a trajectory of persistent heavy smoking with a declining probability of drinking (9%). The sixth class was described by temporary reduction in smoking during pregnancy with high probability of drinking (6%). Covariates differentially predicted class membership, e.g., having a high school degree but not further education predicted concurrent drinking and smoking, and breastfeeding for more than 6 months is protective against concurrent use. Prior to conception, during prenatal care, and in post-natal clinical visits, whether for personal or pediatric care, screening women of reproductive age via characteristics that predict heterogeneity in smoking and drinking trajectories may help guide prevention and treatment options.