Deficiencies of urea cycle enzymes are rare metabolic disorders. Inadequate function of these enzymes may in worst cases lead to hyperammonemic encephalopathy and death. The danger of urea cycle enzyme deficiencies is that previously healthy adults with no prior medical history suggesting these deficiencies may suddenly develop life-threatening complications during prolonged catabolic situations such as delivery or surgery. Since most of the metabolic disorders are diagnosed during childhood, it may sometimes be difficult to bear in mind these rare diseases as a cause of unconsciousness in adulthood. However, early diagnosis and prompt initiation of ammonia-lowering treatment are essential for survival of these patients. We present two pregnant women with urea cycle disorders: one with a known deficiency and an uncomplicated outcome, and another with a previously undiagnosed disorder and life-threatening course of the postpartum period.