By next spring the Norfolk medical team that produced the nation's first test-tube baby last December expects to see its offspring reach a total of 13.

To those who were surprised when the Norfolk group outdid all the country's major medical centers the number may seem almost unreal. But the Norfolk feat, impressive as it is, is only part of an astonishing explosion of in vitro fertilization -- the scientific name for test-tube conception -- now practiced on three continents.

A survey last week showed:

* Fifty-four children have now been born thanks to the efforts of Drs. Patrick Steptoe and Robert Edwards, the Britons who have led the world in laboratory fertilizations since the birth of Louise Brown, the first test-tube baby, on July 25, 1978. Another 37 of their patients are now pregnant.

* Thirty-three babies have been born in Australia and at least 23 more patients are pregnant following the work of two Melbourne medical teams.

* Altogether, at least 150 test-tube babies will have been born by next spring, making yesterday's miracle commonplace. All will be products of conception in a laboratory dish, then, after a few days' growth, normal development in their mothers' wombs.

All will have been born to parents who spent years trying to overcome infertility. Opposition to the technique by those who opposed "interference with nature" generally has faded.

More medical centers are getting into the business. They include Yale and Vanderbilt universities, the University of Pennsylvania, the University of Texas in both Houston and San Antonio, and the University of Southern California, where there has already been one successful birth.

Two Washington-area physicians, Drs. Mark Geier and John Young, have started what may be the country's first privately sponsored program at their Bethesda clinic. They report that they have already made three attempts, so far unsuccessful, at starting the in vitro (literally, "in glass") process by which male sperm cell and female egg cell are joined in the laboratory.

Their early lack of success is not surprising. There has been a learning period for every group.

But the success rate in achieving pregnancies at the three centers -- in Britain, Australia and Norfolk -- is now between 15 and 20 percent per try. This approaches nature's own success rate, for a couple trying to conceive normally has only about a 1-in-4 chance in each month.

"We're surprised and pleased," Dr. Howard Jones, head of Norfolk's Eastern Virginia Medical School team, said last week. "When we started in March 1980 we said it would probably take us three years before we could even evaluate whether it was worth proceeding. The British had to try for 15 years before their first pregnancy. The Australians tried for eight years."

It was just three days after Christmas that the Norfolk group saw its first birth. Some scientists elsewhere, aware of the extensive knowledge and skills needed, wondered if the Norfolk success was a fluke.

It wasn't. The past three weeks have seen the birth of the fourth, fifth and sixth "Norfolk babies." Another three could be born by the end of the year, and four more early next year.

It is tempting to say it is Drs. Howard and Georgeanna Jones, a white-haired husband-and-wife team, who made the difference.

In 1978, then aged 67 and 65, respectively, they retired from their practices and appointments at Johns Hopkins University in Baltimore to help launch a new shoestring medical school operated by the new Eastern Virginia Medical Authority. They brought Norfolk two lifetimes of experience in obstetrics, gynecology and fertility management.

But they, and the school's founders, also helped assemble a small but respected team of experts in genetics, endocrinology, immunology, embryology and reproductive biology. They also were able to draw on the accumulated British and Australian knowledge.

They have added to it, as they will report to some 200 doctors and scientists attending a three-day In Vitro Fertilization Workshop opening in Norfolk today.

How do the Norfolk doctors make a test-tube baby?

* They give the intended mother fertility-producing hormones, partly to make her ovulate at a predictable time, partly so she may produce two or more egg cells or ova rather than the single ovum most women manufacture each month.

* They watch her hormone levels to catch the ripe ova at just the right time.

* Then they remove the ova from the prospective mother through a laparascope, a long, telescope-like tube inserted through the abdomen. This "getting the egg" is one of the most crucial, and, for beginners in the process, often one of the most difficult steps. At Norfolk and the other leading centers, it is now successful 90 percent of the time.

* The egg or eggs -- sometimes two, three or four and, sometimes, even five -- are incubated in a warm, moist chamber with a high carbon dioxide content (like the interior of the human body) to imitate the period they would continue to develop in nature while awaiting the male's migrating sperm. The usual incubation period is six to eight hours. Immature-looking ova may be incubated for a few days to reach maturity.

* Then all the eggs and fresh sperm from the husband are joined in the central wells of clear plastic dishes so the eggs can be fertilized. Here too the success rate is 90 percent.

* Once an egg cell is fertilized, it starts dividing and within 42 to 46 hours has become a four- to six-celled aggregate that is now inserted into the mother's womb.

If more than one egg has been fertilized, all are inserted. The implantation of multiple eggs accounts for much of the increasing success on all three continents. It also accounts for the fact that some test-tube babies have been twins (the Norfolk group has one set of twins awaiting birth).

Still, the process of "implantation" -- the natural way in which the clump of cells attaches itself to the womb and then stays attached -- remains the most difficult hurdle. More often than not the egg or newly developing fetus is lost.

"This is normal expectation," the same thing that happens in nature, Jones said. Many women unknowingly become pregnant for a matter of days, then lose the pregnancy without even realizing they've had one. But at Norfolk and similar centers usually unrecognized pregnancies are detected by close hormonal monitoring.

The high failure rate in this last step means some women have had to try again and again for a success. "One of our patients tried four times and was ultimately successful," Jones reported. But repeated tries require a large investment of time, emotion and money.

At Norfolk today, a couple must pay up to $1,400 in doctors' fees and $3,450 in laboratory and hospital charges for a completed pregnancy and almost as much for each repeated attempt.

In Bethesda, in a set of offices and laboratories on Old Georgetown Road, Drs. Geier and Young foresee only slightly lower charges for their new "Medically Assisted Pregnancy" program, recently advertised by blue-and-white brochures to some 700 area doctors. "But we won't charge any fees for our own services until we've had a number of pregnancies," said Geier, who holds a Ph.D. in genetics as well as an M.D. "The Joneses did not charge either, at first, and we're still learning."

Geier and Young are adopting the Norfolk methods, though Jones said he is not certain they will work "at this stage" without a larger medical team.

Geier and Young said they are convinced, however, they can establish what they call "the first in vitro fertilization service in the private medical world." They said the method will have to become a standard part of private medical practice if many of the hundreds of thousands of infertile couples in this country are to be helped.

All parties involved would like to see medical insurance plans cover the procedures. The Maryland and Washington-area Blue Cross-Blue Shield organizations have not yet acted on any of the Geier-Young claims and Blue Cross-Blue Shield of Virginia has declined to pay for the Norfolk procedure on the grounds that it is still "experimental."

That question was addressed by the American Fertility Society, the organization of doctors and scientists, early this year. The method, the society declared, must now be recognized as "acceptable" in "appropriate patients" at "appropriately staffed and equipped institutions that have demonstrated proficiency and success."