A medical-surgical team at Children's Hospital National Medical Center has successfully separated a pair of conjoined, or so-called Siamese, twins flown here from Italy.

The twin girls now 13 weeks old, are listed in critical condition in the hospital's intensive care unit, where they have been since the completion two weeks ago of the 10 hours operation involving seven surgeons, five anesthesiologists, seven nurses and a staff of 13 consulting specialists.

According to Dr. Judson Randolph, the operation was the first attempt ever made to separate a pair of Siamese twins as thoroughly joined as the two girls, who were one from shoulder to waist, with two heads, four arms and two legs.

"They're quite hearty now," said Randolph. "Their hearts and lungs are strong.But until we see healing" of the gaping wounds in their upper torsos, they will be listed in critical condition.

"There was only one previous case of twins conjoined like this surviving," said Randolph, who headed the surgical team. That pair born in Russia, was never separated and lived "for some years."

Randolph said that when the twins arrived at Children's at eight weeks of age "we were disappointed that they were" more closely joined than had been anticipated. "We felt their chances were slim," he said.

So-called Siamese twins are identical twins - twins formed from the fertilization of a single ovum by a single sperm - who fail to separate completely. Under normal circumstances identical twins result from the breaking in two or an egg, but if that break is only partial, conjoined twins result.

It is estimated that one in 80,000 to one in 200,000 births produces a pair of live or dead conjoined twins. However, very few of the pairs live and there are only about 400 recorded incidents of live births of such twins in medical history.

Such twins are called Siamese because P. T. Barnum, the 19th century showman-huckster, found a Chinese-born pair in Siam named Eng and Chang and brought them to this country where he displayed them as a sideshow attraction.

Like 40 per cent of conjoined twins, Eng and Chang were joined at the sternum, the bone at mid-chest, diaphram, upper abdominal wall and navel.

Twins joined in that manner have, in the recent past, been successfully separated.

The twins at Children's Hospital, however, represented a far more difficult task for the surgeons. They each have a heart, lungs, stomach, one - rather than two - kidneys and shared a liver, which was divided during the surgery.

Because they had completely separate circulatory systems they each required separate anesthesia during surgery.The surgeons, anesthesiologists and nurses were broken into two teams, the blue and the red, and each team was responsible for one twin.

"One hour into the operation one of the twin's hearts stopped," said Randolph.

What made the operation enormously complicated was that in separating the twins the surgeons created huge wounds in each of their upper torsos and had to use plastic covering to close the wounds.

Randolph said that the fact that each twin only has one leg should not prove to be a serious medical problem. Because they will be one legged virtually from birth they will develop extraordinary agility and balance, he said. It may be possible when they are older to fit them with artificial legs.

The twins were brought to the attention of Children's by Dr. Enrico Davoli, a McLean. Va., pediatrician with friends in Italy.

After receiving a special waiver from the federal Civil Aeronautics Board, Trans World Airlines agreed to fly the twins and their parents from Italy to Washington free.

The family is not being charged for the infants' care at Children's and a hospital spokesman said yesterday that the hospital bill alone, excluding what the surgeons would usually charge, is about $34,000.

Randolph said that in addition to the medical problems, the surgical team was faced with "ethical questions which the whole staff" was involved with. "We had to decide whether the risk (of surgery) should be taken. It was an enormous risk and these were two healthy twins."

It was decided, he said, that it would be "cruel" not to separate them. And, "these were two quite separate little people, which made it impossible to consider" sacrificing one to save the other.

"They laughed, smiled and cried independently," said the surgeon, who said that "if we had chosen to save one child it would have been a simple procedure. There would have been plenty of skin and soft tissue" to close the wound.

"The ethical questions were enormous," he said, "but I think they were approached correctly."

The anesthesiology problems were as complex as the surgical problems. The individual heart rates, blood pressure, body temperature and other vital signs of each twin were constantly monitoried during the operation.

According to a hospital spokesman, circulating hot water matresses were used, in addition to infra-red heating lamps, to keep the infants warm during the prolonged session in the operating room.

Althought the twins parents have not been indentified, their mother prepared a statement in Italian which was translated for the press by father Charles Zanoni, a local Roman Catholic priest.

"I was bringing with me to the United States two little jewels, two beautiful babies, who had been marked by a cruel destiny. As I looked at them I cried, thinking I would never be able to enjoy their beauty," she wrote.

When told the babies might die during surgery, the mother wrote, she "was willing to take this risk . . . because this was the only thing left for them to have any possible future.