BACKGROUND Smoking during pregnancy has adverse health consequences for the mother and fetus. E-cigarettes could aid with smoking cessation but there is limited research on the prevalence and patterns of e-cigarette use, and their association with smoking cessation among pregnant smokers. METHODS We conducted a secondary analysis of a randomized controlled trial of a text-messaging program for smoking cessation among a U.S. national cohort of pregnant smokers (n = 428). Outcomes assessed were trajectories of e-cigarettes use from baseline to one-month follow-up, and longitudinal association between e-cigarette use at baseline and smoking cessation at one-month follow-up. RESULTS At baseline, 74 (17.29%) pregnant smokers used e-cigarettes in the past 30 days and 36 (8.41%) used e-cigarettes in the past 7 days. The primary reason stated for using e-cigarettes during pregnancy was for quitting. E-cigarette use between baseline and 1-month was inconsistent. Of 36 dual-users at baseline, 20 (55.56%) stopped using e-cigarettes by the 1-month follow-up and 14 initiated e-cigarette use. There was no evidence of an association between e-cigarette use at baseline and the primary smoking cessation outcome, 7-day point prevalence abstinence [adjusted odds ratio = 0.79, 95% confidence intervals = 0.33-1.92]. CONCLUSIONS A secondary analysis of a national sample of pregnant smokers indicates that use of e-cigarettes is inconsistent and is not associated with improved smoking cessation outcomes. There is an urgent need to further examine the risk and benefits of e-cigarette use, especially during pregnancy.