University of Maryland Hospital surgeons who separated a Baltimore couple's Siamese twins last month said today that the infants were in good health and would be ready to leave the hospital in about a week.

At a press conference, doctors displayed the 3-month-old girls and described the rare and intricate operation in which they were separated. The infants had been joined at the chest and abdomen.

"It went like absolute clockwork," said Edward G. O'Laughlin, the hospital's director of pediatric anesthesia. "The children were totally stable throughout the operation."

The 7 1/2-hour surgery, performed on a Sunday when it would conflict least with other scheduled operations at the hospital, involved six surgeons, six anesthesiologists and 12 nurses.

The team worked in a crowded operating theater, where temperatures were raised deliberately to 85 degrees to provide a warm environment for the babies, O'Laughlin said.

Hospital officials said Siamese twin separation is unusual and often unsuccessful. There are only about 350 Siamese, or "conjoined," twins reported in medical history, they said.

Most of the twins die at birth or shortly thereafter, the officials said, and successful surgery on those surviving more than a few months has been spotty.

There have been three previous cases of Siamese twins in Baltimore in the last half century, hospital officials said. Two sets died. One set was successfully separated at Johns Hopkins Hospital in 1982 and was reported still living in the Baltimore area.

Officials refused to identify the twins displayed at University Hospital today, saying the family requested anonymity. They said only that they were the mother's first children and that she was under 35.

The twins were born 6 1/2 weeks prematurely on April 20 and weighed a total of 7 pounds, 9 ounces. Doctors had detected five months before birth that the fetuses were joined at the chest and stomach and carefully monitored them to their delivery by cesarean section.

Through tests, doctors determined that the twins had a common liver but separate hearts, lungs, gallbladders, kidneys, stomachs and intestines, making separation surgery possible.

In fact, the liver, while shared, was large enough so that it could be cut in about half, and each child could be left with a working organ, doctors said.

A team of surgeons and anesthesiologists began preparing for the separation weeks in advance, consulting with specialists and even doing mock drills with dolls.

During surgery, the liver separation was a chief concern because of potential blood loss. But even more critical, doctors said, was closing the huge gaps in the chest and abdominal areas after the separation.

For the first five days after the operation, the twins were in critical condition because of their incomplete chest walls and the tightness of the closures around the hearts, lungs and stomach organs.

Gradually, said Dr. J. Laurance Hill, the hospital's chief of pediatric surgery , the babies became stabilized and were weaned from ventilators used to help them breathe.

For the first two weeks, they were fed intravenously. Then they graduated to tube feeding and three days ago began bottle feeding.

Each baby now weighs about 6 pounds. They are both in "good health," said Hill, although it is impossible to say now how they will fare in the future.

The hospital has not yet calculated the cost of the care and surgery for the twins, "but it is certainly far beyond what any ordinary family or its insurance would cover," said one official.

"We're going to have to absorb some of the cost," said hospital spokeswoman Judy Cramer, "but the hospital is not really worried about that."

Cramer noted that a stay in the hospital's neonatal intensive care unit, where the twins have been since birth, is $1,000 a day.