A young woman in a laboratory coat sits at a desk cutting out what appear to be photographs of striped sausages.

Meticulously she matches them up on a numbered chart. Two of the striped sausages, with matching bands, line up under each number.

Under the number "21" there are three.

The young woman sighs. She is a cytogeneticist, analyzing photographs (magnified hundreds of times) of the sausage-shaped chromosomes taken from a single cell of a developing embryo. The cell itself was isolated from a small amount of amniotic fluid drawn earlier from the mother.

The extra 21st chromosome is a dead giveaway that the embryo has Down's syndrome and will develop with varying degrees of mental retardation and other abnormalities.

At this point there is just the chart with the extra chromosome. The anguish will come later. (Extra or missing chromosomes always portend serious problems. Normal humans have 46.)

Down's is one of a growing number of genetic defects that can be detected prenatally through chromosomal analysis. Others, like neural-tube defects--anencephaly and spina bifida, for example--are detected biochemically. Some potential abnormalities are limited to certain ethnic groups: Tay Sachs disease among Jews; sickle cell anemia among blacks. (Still others, like cystic fibrosis, a major killer of children and young adults, cannot yet be detected prenatally.)

These findings are all part of a relatively new medical speciality: genetic counseling. And genetic counseling goes hand in hand with another burgeoning field: the treatment of infertility.

Today, the ultimate treatment for infertility is the implantation in the mother of her own egg, fertilized outside of her body with her husband's sperm--the so-called in vitro fertilization (IVF). But that process is still considered a last resort, performed in only a few specialized centers, after all other techniques have failed.

Indeed, the triad of genetic counseling, infertility treatment and IVF has been almost exclusively the domain of research centers, usually connected to universities or teaching hospitals.

Dr. John Young, obstetrics-gynecology, and Dr. Mark Geier, M.D. and Ph.D. (in genetics), are breaking the mold. Their practice--they call themselves "Genetic Consultants"--is basically clinical, which means they are concerned principally with the patient, rather than in furthering research. Their specialties are twofold: genetic counseling and the treatment of infertility, up to and including IVF. As far as they know, they have the only IVF program in the Washington area.

"We regard them [their specialties] as our two children," says Young, "and we have no favorites."

The men met nearly five years ago when both were training at Johns Hopkins Medical School, a major center for genetics and infertility research. Drs. Georgeanna and Howard Jones, the Norfolk endocrinologist and obstetrician-gynecologist team behind this country's first IVF baby, had spent 35 years at Hopkins working on infertility and were--and continue to be--a major influence on Young and Geier.

Says Young, "Soon after I opened my practice, one of the local obstetricians suggested to me that since my recent training at Hopkins allowed me some areas of expertise not commonly found, I ought to think about a genetic counseling service . . ." Young contacted Geier (who got his Ph.D. and M.D. degrees at George Washington University) and the two quickly found that they were meeting a major community need.

As their practice burgeoned they felt the need for their own laboratory and became part owners of Molecular Medicine, possibly the most sophisticated privately owned facility of its kind.

Except for the laparoscopy, and any needed microsurgery that is performed under a general anesthetic in a hospital, all testing, analyses, sonography, amniocentesis and even the delicate in vitro fertilization procedures are performed either in the office or in the lab, a few steps away. The Bethesda practice is the largest of its kind in the state of Maryland. They perform, for example, more amniocentesis procedures than Hopkins itself; in fact, say the doctors, they have become, in the three years of their existence, twice as big as the Hopkins facility.

For the most part, patients referred to them (by about 200 area obstetricians) are sent back to the referring physician after consultation, treatment and often pregnancy, thanks to the latest techniques in treating infertility.

The Genetic Consultants IVF program has just started. So far the physicians have implanted eggs in two patients, but have not yet produced a pregnancy through IVF.

The major portion of their practice today involves genetic counseling in the four major categories of inherited disorders:

* Chromosomal abnormalities of which Down's syndrome is the most common, occurring more frequently in pregnancies after age 35. A 24-year-old woman has a less than 1-per-1,000 chance of having a Downs baby whereas a woman at 40 has one chance per 100 births.

* "Single gene" disorders caused by a mutation in a single gene. Although usually too small to be seen directly, scientists now can detect some of the abnormalities by biochemical tests, in some cases even before conception. Examples are Tay Sachs disease and sickle cell anemia.

* Sex-linked disorders like hemophilia or Duchenne muscular dystrophy. Most are disorders linked to the X chromosome and generally are carried by the female and afflict the male. Females have two X chromosomes; males have one X and one Y.

* So-called "developmental abnormalities" of which a common one is the neural-tube defect, a condition where the central nervous system develops incompletely. One neural-tube defect is spina bifida, in which there are openings along the spinal column often resulting in varying degrees of paralysis and mental retardation. In its most severe form the defect is called anencephaly, a condition, almost always fatal, in which the brain develops incompletely, or not at all.

Genetic Consultants is the first private service licensed to perform the alpha-fetoprotein test, a series of steps that can pinpoint neural-tube defects in the two out of 1,000 cases where they occur. Because most occur in women with no previous family history of the disorder, Young and Geier believe all pregnant women should be screened at the proper time in the pregnancy--between the 15th and 21st weeks.

The AFP kit has not yet been fully approved for use in this country. The Food and Drug Administration, however, has permitted experimental use, and now is beginning to allow the test where there is appropriate follow-up.

Both physicians emphasize that no panic, no conclusions (and certainly no decision on abortions) be drawn from any of the screening tests until after the amniocentesis.

"You don't do a hysterectomy on the basis of one abnormal Pap smear," notes Geier. "You wait until everything else--the majority of things--is ruled out first."

Chromosome testing, says Geier, can rule out several hundred disorders in addition to Downs. The alpha-fetoprotein test can pick up about 20 more. At the moment, however, only one biochemical test can be run at a time and not all the tests can be run, even separately in the same place.

"It is," notes Geier, "the world's knowledge." He is currently working on techniques for multiple biochemical testing with elaborate new equipment available at the National Institutes of Health, where he is a guest scientist. Meanwhile, the biochemical testing is done only to pinpoint a single suspected problem--Tay Sachs, for example, or sickle cell anemia.

Most of the major genetic defects are relatively rare occurrences, accounting for 3 to 5 percent of the approximately 3 million annual births in this country. However, says Geier, they claim an inordinate share of America's health-care expenditures--perhaps up to 50 percent.

"Our institutes are filled with people who have various defects who are kept there for life . . . Taking care of these children is one of our biggest health-care expenditures. Yet almost zero is spent in studying how to prevent or correct these kinds of things."

There is virtually no grant money available, he points out, and "NIH, the greatest research place in the world, has no unit whatsoever that is obstetrical.

"I'm not just talking about possibly controversial genetics research," he says passionately, "but I mean things like how to take care of babies as they are being born . . . or the best way to do a Caesarean section.

"The major improvements in medicine are not things like the artificial heart. The advances that lengthen the life span are mostly in pediatrics and obstetrics."