The pregnant woman is susceptible to a variety of respiratory complications. When a pregnant patient presents with an abnormal chest x-ray or a pulmonary complaint, an understanding of the pathophysiology of pregnancy will guide the clinician in establishing a diagnosis. Pregnancy brings about many changes to a woman's body. One of the more intriguing is a decrease in the T helper cells, resulting in a state of relative immunosuppression. Despite this, the prevalence of infectious pneumonia is not increased in pregnancy. Complications from pneumonia, however, are increased in the pregnant host. Most notably are increases in both mortality related to influenza infection and the risk for dissemination of coccidioidomycosis. Other physiologic changes predispose the pregnant woman to certain disease processes. Hypercoagulability associated with pregnancy results in a marked increase in the incidence of thromboembolic disease. Although rare, pregnancy is also associated with other embolic phenomena including amniotic fluid embolism, air embolism, and trophoblastic embolism. Because of the increases in intravascular volume and cardiac output that occur in pregnancy, women with underlying structural heart disease will frequently present for the first time or have an exacerbation of their disease. This is especially true of mitral stenosis. Peripartum cardiomyopathy also can occur, and for the majority of patients, the heart remains damaged for life. Finally, although uncommon, lymphangioleiomyomatosis will often present or become exacerbated during pregnancy. Patients with this disorder need to be counseled concerning the increased risk associated with pregnancy. This paper reviews the various respiratory complications associated with pregnancy.