Thirteen pregnancies in ten women on oxcarbazepine (OXC) monotherapy and one pregnancy in a woman on OXC and topiramate therapy were retrospectively analyzed. A significant decrease of ratio plasma concentration of 10-monohydroxy derivate (MHD) of oxcarbazepine to dosage was found by 26.2% during first trimester, by 36.5% during second trimester and by 38.2% during third trimester when compared to pre-pregnant levels. Eight patients experienced seizure deterioration during the pregnancy, five of which had been seizure-free before the pregnancy. In seven pregnancies (50%) the seizure frequency was at least doubled during pregnancy compared to a pre-pregnancy baseline. There was a trend toward a correlation between seizure deterioration and decrease in plasma concentration of MHD.