Background Data on the effects of extremely low frequency electromagnetic fields (ELF-EMF) on pregnancy outcomes are inconclusive. Objectives To study the relation between maternal cumulative exposure to ELF-EMF during pregnancy and the risk of prematurity or small for gestational age in a pooled analysis of two French birth cohorts. Methods Elfe and Epipage2 are both population-based birth cohorts initiated in 2011 and included 18 329 and 8400 births respectively. Health data and household, mother and child characteristics were obtained from medical records, by face-to-face interview at maternity, and completed during follow-up. A recently updated job-exposure-matrix (JEM) was used to assess cumulative exposure to ELF-EMF during three periods: 1) until 15, 2) until 28 and 3) until 32 weeks of gestation. Analyses were restricted to single live births in mainland France and to mothers with documented jobs (n=19,894). Differences in selection frame between the two cohorts were controlled using a propensity score weighting method. We used multiple imputation method to deal with missing data. Logistic regression models adjusted for the main potential confounders were used. Results According to the period studied, 3.2% to 4% of mothers were classified as highly exposed to ELF-EMF. An increased risk of spontaneous prematurity was observed among the most exposed groups in period 2 and period 3. Overall, no consistent association with small for gestational age was found. Conclusion This study, with substantial statistical power provides evidence of a possible association between cumulative exposure to ELF-EMF during pregnancy and the risk of spontaneous prematurity.