Eisenmenger syndrome or Eisenmenger's syndrome is defined as the process in which a long-standing left-to-right cardiac shunt caused by a congenital heart defect (typically by a ventricular septal defect, atrial septal defect, or less commonly, patent ductus arteriosus) causes pulmonary hypertension and eventual reversal of the shunt into a cyanotic right-to-left shunt. Because of the advent of fetal screening with echocardiography early in life, the incidence of heart defects progressing to Eisenmenger syndrome has decreased.

Eisenmenger syndrome in a pregnant mother can cause serious complications, though successful delivery has been reported. Maternal mortality ranges from 30% to 60%, and may be attributed to fainting spells, blood clots forming in the veins and traveling to distant sites, hypovolemia, coughing up blood or preeclampsia. Most deaths occur either during or within the first weeks after delivery. Pregnant women with Eisenmenger syndrome should be hospitalized after the 20th week of pregnancy, or earlier if clinical deterioration occurs.