Several teams of doctors hurriedly assembled on the night of Aug. 20, after an echocardiogram showed that Ms. Errante, who was complaining of chest pain, had a dissected aorta, a tear in the inner lining of the main artery leading from the heart.

"It's a disease that we have a great deal of respect for," said Dr. Frank C. Seifert, a cardiothoracic surgeon at Stony Brook University Hospital, which held a news conference on Monday to describe the dual operations to reporters.

The condition led to the formation of a bubble of blood between the layers of Ms. Errante's aorta, which Dr. Seifert said could have either burst or blocked blood flow through other major arteries. Untreated, half of all people with aortic dissections die within the first 48 hours of the condition developing. And 95 percent are dead within a month, he said.

While the condition is rare, it has been linked to pregnancy, and Dr. Seifert said he had repaired such a tear in two pregnant women, although in neither of those cases was the fetus old enough to be delivered before surgery. In one case, he said, the fetus remained viable and was delivered at full term; in the other, the fetus was stillborn.

What made Ms. Errante's situation unusual was the presence of triplets, and the need to deliver them immediately. Dr. Seifert explained that during the surgery, Ms. Errante's body had to be cooled and her heart stopped for 90 minutes, conditions that could have killed the fetuses.