Physiologic changes during the last trimester of pregnancy include reduced functional residual capacity and residual volume, increased alveolar-arterial difference for oxygen, and in the supine position reduced cardiac output. In conjunction with sleep-related apnea or hypoventilation, these could lead to maternal oxygen desaturation during sleep. Because we could not find detailed respiratory sleep studies in late pregnancy, we performed complete polysomnography on 6 pregnant women at 36 wk gestation and again postpartum. Contrary to what we expected, oxygenation was well maintained, with neither mean nor minimal arterial oxygen saturation significantly different during pregnancy. Frequency of apnea and hypopnea and total apnea and hypopnea time were significantly reduced during pregnancy. This finding may be due to increased levels of progesterone during pregnancy and may contribute to preservation of maternal oxygenation during late pregnancy.