This was a low risk pregnancy (G1P0) first time seen at 15 weeks. Ultrasonographic examination of the fetal heart showed:

- a thin membrane dividing the left atrium into proximal (venous atrium) and distal chamber, the vestibule - cor triatriatum;
- secondary dilation and congestion of the pulmonary venous tree.

The membrane disappeared at 16 weeks. The rest of the pregnancy was uneventful and the neonate was born with normal heart.

The separation between the venous atrium, a remnant of the pulmonary venous sac and the vestibule (near the mitral valve) is occasionally seen on the basal side of the foramen ovale, in second trimester fetuses. As long as the fenestration is sufficient and there are no hemodynamically signifant obstruction the condition is usually not detected.

Image 1 and videos 1, 2: 15 weeks of pregnancy; the image and videos show four-chamber view of the heart with a thin membrane within the left atrium dividing the atrium into venous or proximal (pc) and vestibular or distal (dc) chamber - cor triatriatum.