Thai doctors announced Thursday that they successfully performed a rare operation to separate a pair of conjoined female infants, teasing apart their hearts and livers in the delicate procedure.

The girls, Panwad Tiyenjai and Pantawan Tiyenjai, were so-called thoracophagus twins -- with their bodies joined from chest to abdomen, said an announcement from Bangkok's Siriraj Hospital.

What made their case unusual was that the upper chambers, or atriums, of their hearts were connected, with one pumping blood to the other, said Dr. Somchai Sriyoschati, a cardiac surgeon who took part in the operation.

The 12-hour operation on Feb. 20, in which 61 doctors and nurses took part, left both girls in good condition, though one has a slight heart defect which can be fixed when she gets older, he said. The girls were eight months old when they were operated on.

The hospital did not say why it delayed in announcing the operation.

Somchai told The Associated Press that a CAT scan and an MRI test before the operation clearly showed the connection between the two hearts, but doctors were unsure if one was dependent on the other.

In some cases where hearts are joined, a diagnosis results in the conclusion that the hearts cannot sustain the patients separately, and that the life of one patient might have to be sacrificed to let the other, stronger one live.

When the operation began and the hearts were exposed, doctors blocked the flow of blood at the joining point to see if each heart could operate independently, Somchai said. Only after finding that stopping the flow caused no apparent problem, did they proceed, he said.

Had stopping the flow between the two hearts caused a problem, further surgery would have been terminated and the twins left in their conjoined state, he said. Conjoined twins who share a heart are often more vulnerable to sickness and have a poor life expectancy compared to separated ones.

The hospital's claims to having performed the first successful operation of its kind -- where the hearts were joined at their upper chambers -- could not immediately be confirmed.

It was not clear how it differed from an operation performed in the United States in 2002 at the University of Maryland Hospital for Children, where doctors successfully separated two Ugandan girls, Loice and Christine Onziga, who also had connected livers and hearts connected by their upper chambers.

Conjoined twins form when an embryo begins to split into identical twins but stops part way, leaving the partially separated egg to mature. They occur once in every 150,000 to 200,000 live births and are three times as likely to happen to females than males.

The chances of conjoined twins surviving depends on how they are connected. About 40 to 60 percent are stillborn and 35 percent survive 24 hours or less. Those who survive longer are often plagued by medical complications due to shared organs and vital systems.