OBJECTIVE This study was undertaken to (1) measure the frequency of AED monotherapy or polytherapy during pregnancy and AED discontinuation prior to pregnancy in a cohort of women with treated epilepsy; and (2) describe the frequency of major congenital malformations according to maternal use of AED during pregnancy. METHODS A cohort of epileptic pregnant women was identified within the Quebec Pregnancy Registry and was divided into three groups based on maternal AED use during pregnancy: AED monotherapy, AED polytherapy and no AED use. RESULTS Of the 349 pregnancies meeting eligibility criteria, 79.6% were exposed to AED monotherapy and 5.8% to polytherapy during pregnancy; 14.6% discontinued AED prior to pregnancy. The most commonly used AEDs were carbamazepine (29.9%) and valproic acid (19.7%); the most common AED polytherapy combination was carbamazepine combined with clobazam (2.5%). Of 111 deliveries in the group of women on monotherapy during pregnancy, 9.9% (n=11) were born with major congenital malformations; in the group of women treated with polytherapy, 19.0% (n=8 over 42) of babies had major congenital malformations compared to 20.0% in women who discontinued AEDs prior to pregnancy. CONCLUSIONS This study demonstrates that the majority of women suffering from epilepsy were treated with monotherapy rather than polytherapy during pregnancy. While most used other agents, an important number of women continued to use valproate in pregnancy despite the long standing evidence of its teratogenicity and increasing evidence of its neuro-toxicity to the fetus.