Pregnancy predisposes women to four specifi c infectious complications: pyelonephritis, chorio amnionitis (including septic abortion), endometritis (often following Cesarean delivery), and pneumonia. Pyelonephritis results from colonization of the kidney with Gram-negative bacteria secondary to loss of ureteral sphincter tone associated with progesterone. Pneumonia results, at least in part, from aspiration of gastric contents as a consequence of loss of lower esophageal sphincter tone and diaphragmatic elevation. Patients are also at elevated risk for viral and fungal pneumonia due to pregnancy-induced immunosup pres sion. Chorioamnionitis results from altera tions in the pH and increased glycogen content of the vagina, resulting in loss of the barrier for bacterial entry. It may complicate chorionic villus sampling, amnio centesis, or attempted instrumental (septic) abortion.