The pregnant patient in the intensive care unit (ICU) poses many challenges. Physicians and nurses need to understand the basic concepts of maternal physiology and fetal physiology. The physicians must care for the mother and consider the potential adverse effects that diagnostic and therapeutic interventions might have on the fetus. This article focuses primarily on the normal pregnant state with appropriate references to how pregnancy may affect the patient in specific circumstances relating to the ICU. The physiologic adaptation of the major organs to the pregnant state should be familiar to ICU physicians.