Objective The aim was to estimate the proportion of women who reported cannabis use during pregnancy, to analyse the demographic and social characteristics of users, and the link between cannabis use and either preterm or small‐for‐gestational‐age birth. Design Data were obtained from interviews of a representative sample of women giving birth in France in 2010 in the days after delivery, and from their medical records. Setting All maternity units in France. Sample The analysis includes women with live singleton births in metropolitan France who responded to the question about cannabis use during pregnancy: in total, 13 545 women. Methods The percentage of cannabis users during pregnancy was estimated, and variations according to social characteristics were described. Logistic regression analyses were used to investigate any associations between cannabis use and preterm birth or small‐for‐gestational‐age status. Main outcome measures Percentage of cannabis use, preterm birth rate, and small‐for‐gestational‐age rate. Results In all, 1.2% of women reported having used cannabis during pregnancy. This percentage was higher among younger women, women living alone, or women who had a low level of education or low income. It was also associated with tobacco use and drinking alcohol. Cannabis users had higher rates of spontaneous preterm births: 6.4 versus 2.8%, for an adjusted odds ratio (aOR) of 2.15 (95% CI 1.10–4.18). The corresponding aOR was 2.64 (95% CI 1.12–6.22) among tobacco smokers and 1.22 (95% CI 0.29–5.06) among non‐tobacco smokers. Conclusions Although the reported rate of cannabis use during pregnancy in France is low, efforts should be continued to inform women and healthcare providers about the potential consequences of its use.