PURPOSE Isolated cor triatriatum sinistrum (CTS) is a heart malformation in which a perforated fibromuscular membrane divides the left atrium into two chambers. When communication between these chambers is restricted, a patient may have signs and symptoms of mitral stenosis. The later stages of pregnancy are associated with tachycardia and increases in intravascular volume. We describe how this altered physiology may affect pregnant women with asymptomatic CTS. We also review the literature relating to pregnancy in patients with CTS. CLINICAL FEATURES A 30-yr-old primigravida, at 40 weeks of gestation with pre-pregnancy diagnosed asymptomatic CTS, was admitted for delivery. She had no cardiac symptoms during pregnancy, and her vaginal delivery under epidural analgesia was uneventful. This cardiac malformation is infrequently described in pregnant women, but a literature review showed that the physiology of late pregnancy with increases in hemodynamic variables may result in cardiac decompensation. CONCLUSION While our patient with isolated CTS and an unrestrictive intra-atrial membrane had an asymptomatic pregnancy and an uneventful labour, the literature review suggests that the increase in intravascular volume and heart rate that occurs during late pregnancy and after delivery may result in cardiac decompensation, even in patients with asymptomatic CTS.