A rare method of test-tube conception known as the frozen embryo process resulted in twins for a Reston woman at Loudoun Memorial Hospital this week in what is believed to be a first in this country.

The 36-year-old woman, who had tried to get pregnant for nearly four years, delivered a 7-pound, 4-ounce girl and an 8-pound, 2-ounce boy by cesarean section on Tuesday afternoon, officials at the Leesburg hospital said yesterday. The twins were said to be healthy and doing well.

The ability to freeze an embryo -- a woman's egg fertilized with a man's sperm -- is one of the latest developments in the field of in vitro fertilization, which originated in England a decade ago.

The method involves surgically removing eggs from a woman, fertilizing them in a laboratory dish and implanting up to four embryos in the woman's uterus. At least three-fourths of the time the process does not result in a successful pregnancy.

The recently gained ability to freeze and store embryos should greatly improve the odds of an infertile couple having a baby, doctors said, because embryos not immediately implanted can be frozen and implanted later.

The first successful delivery using the frozen embryo process took place in Australia in 1984; the first such baby in this country was born in Los Angeles in June 1986.

Although about 10 frozen-embryo babies have been born in the United States, until Tuesday none had been born in the Washington area. Doctors said it was the first known case of twins being born with this method in this country; similar births have happened in other countries.

Doctors at the Genetics and IVF Institute in Fairfax County, who oversaw the freezing of the Reston woman's embryos, said the introduction of the new technique here should give hope to couples who have been unable to have children. "This means women can have repeated chances of transferring embryos without the risks, inconvenience and costs of going through several cycles," said Dr. Maria Bustillo of the institute, the largest in vitro fertilization clinic in the area and the second largest in the country.

The institute opened its doors in 1984, and has had 174 pregnancies, approximately 80 of which have resulted in successful births, Bustillo said. She said the institute is overseeing nine other pregnancies that are the product of the frozen-embryo process.

The twins' parents asked that they be identified only by their first names, Peggy and Will, and would not take questions from reporters, hospital officials said.

Doctors described the successful pregnancy as an example of the "miracle" of in vitro fertilization technology, ending a four-year ordeal for the couple -- including the treatment of Peggy's cervical cancer.

When the couple realized that medical problems were preventing pregnancy, they went to see Dr. H.R. McLeod, a Leesburg obstetrician/gynecologist, who said he directed Peggy to Georgetown University Hospital for surgery on her blocked Fallopian tubes.

When she still did not become pregnant, Peggy went to the Fairfax clinic, which she had read about in a magazine, according to McLeod. She went through an in vitro fertilization cycle in 1986 -- a two-week period during which she took fertility drugs and had three eggs removed in a special, nonsurgical technique. The resulting embryos were implanted, but there was no pregnancy.

Finally, a year ago this week, doctors at the Fairfax institute retrieved seven eggs from Peggy's ovaries and immediately implanted four of the resulting embryos, freezing the others. When the first four did not develop, doctors implanted the others three months later, and Peggy became pregnant.

The doctors said the birth of twins was not intended, and that normal practice is to implant several embryos because most will not be carried to delivery. "We put in three to get one," said Dr. Edward Fugger, the institute's expert on the freezing process. "The extra was a bonus."

To some of the doctors and hospital officials involved, this week's births underscore how far the technology of in vitro fertilization has advanced. "The technology was not even thought of when I was in training, and it's now come out here to Leesburg, Virginia," said McLeod, who performed the cesarean section on Peggy. "This woman is Everywoman."

STEP 1: Egg is fertilized with sperm in laboratory dish.

STEP 2: Embryo is frozen at temperature of -259 degrees. Up to a dozen embryos are stored at a time.

STEP 3: Later, embryo is thawed, implanted in woman's uterus.