Increasing number of patients with inborn errors of metabolism (IEM) are now reaching adulthood and are in position to reproduce. Because of the rarity of individual disorders our knowledge of risks factors associated with pregnancy is limited. Obstetrics problems in IEM can be divided into two categories: pregnancy effects on maternal metabolic disorders and relation between mother and fetus related to who of them is affected. Detrimental effects upon the fetus may be directly caused by maternal disease, as it occurs in PKU, or indirectly by maternal supplementation with harmful substrate, as occurs in galactosemia. Less commonly, fetal inborn error of metabolism may affect the mother's health. Pregnancies in which the fetus had long-chain hydroxyacyl-CoA dehydrogenase deficiency have been complicated by life-threatening HELLP syndrome (haemolysis, elevated liver enzymes and low platelets) or AFL (acute fatty liver of pregnancy) during third trimester. The management of labor and the postpartum period (for women and newborns) has to be carefully planned to avoid significant metabolic decompensation.