DOCTORS are increasingly offering pregnant women two prenatal tests that are so new that in some respects they are still experimental.
Uncertainties about the safety and accuracy of the tests mean that, more than ever before, pregnant women are being confronted with bewildering, unfamiliar decisions. But the tests, designed to detect serious and common birth defects, are providing what many women regard as welcome new opportunities to learn the condition of a fetus.
One procedure, in which a sample of tissue is removed from the uterus, can detect a variety of genetic defects, including Down's syndrome, as early as nine weeks into pregnancy. But in some cases, the test itself will result in miscarriage; researchers still do not know exactly how dangerous it is compared with the long-used procedure of amniocentesis, which detects the same fetal problems.
The second procedure, a blood test, can indicate the possibility of nervous system defects and Down's syndrome, the most common cause of mental retardation in this country. In itself, the test does not pose a danger to the fetus. But its results are only suggestive, and some critics worry that in follow-up tests, the number of normal fetuses that are lost may be greater than the number of defective fetuses that are detected.
The use of both tests is growing explosively, Federal experts say.
''There's been a real change in obstetricians' attitudes in the past couple of years,'' said Dr. Joseph Schulman, director of the Genetics and IVF Institute in Fairfax, Va., a private fertility clinic. More doctors, he said, see value in the new procedures.
As a result, more women are facing increasingly difficult decisions about which prenatal tests to accept and how to act on their results. Because the tests are so often a prelude to abortion, they have heated an already simmering controversy.
But many obstetricians are offering them anyway, in part because they feel the tests offer undeniable benefits and, at least in the case of the blood test, because they fear their legal liabilty if they do not offer the test. In California, physicians are required by law to offer the blood test to their patients; about half the patients are agreeing.
The first test, called chorionic villus sampling, is offered at about 9 to 11 weeks of pregnancy. Many experts believe it will eventually replace at least two-thirds of the more familiar amniocentesis procedures, which can only be done in about the fourth month of pregnancy, when abortions are more difficult, both physically and emotionally.
Amniocentesis involves cells obtained from the fluid surrounding the fetus in the amniotic sac. The diluted cells must be grown in the laboratory before they can be analyzed, so results are only available after about two weeks.
In chorionic villus sampling the cells are taken from hairlike projections, or villi, on the gestational sac that surrounds the fetus early in pregnancy. The results are available within a few days. With either test, examination of the cells can disclose birth defects like Down's Syndrome, Tay-Sachs disease or others. Competence of Labs Assessed
The second test, alpha-fetoprotein screening, is a blood test that is mainly used to detect defects in the neural tube - which forms the spinal column and brain - the most common birth defects in this country. It can also indicate when a fetus is at greater risk for Down's syndrome.
Dr. James Haddow of the Foundation for Blood Research in Scarborough, Me., which assesses the competence of laboratories performing the test, said his foundation is analyzing results from 130 labs now and that each lab does anywhere from 2,000 to 60,000 tests a year.
As for chorionic villus sampling, the only data are from a registry maintained by Dr. Laird Jackson of Jefferson Medical College in Philadelphia, Dr. Jackson's figures, which understate the true figures, are that 6,544 procedures were reported in the United States last year and 11,754 this year. So far, the test is most commonly offered to women in major urban areas.
The chorionic test was introduced in this country in 1983, but it was at first offered at only a handful of medical centers. Initially, physicians used a plastic catheter, inserted through the cervix, to obtain the samples. The F.D.A. regulated the catheter, which it classed as a new device. But today more and more obstetricians are using a needle inserted into the woman's abdomen, an unregulated technique. Because the F.D.A. cannot regulate procedures, only drugs or devices, it has been unable to limit use of the test. Although more than 30 groups of medical researchers are offering the test as part of a Federal study that meets F.D.A. guidelines, many more physicians are offering it on their own.
Many of them believe the abdominal approach is safer than the trans-cervical procedure. Dr. Jackson suggested that the abdominal technique might offer less risk of hitting vital fetal structures, causing miscarriages. And Dr. George Rhoads of the National Institute of Child Health and Human Development in Bethesda, Md., said he believed that with the abdominal technique there was less of a chance of infection.
A major question has been the comparative safety for the fetus of the chorionic test and amniocentesis. Amniocentesis, performed at about the 16th week of pregnancy, causes fetal death in about one-half of 1 percent to 1 percent of cases, according to the latest studies.
By contrast, in more than 32,000 chorionic tests over the past four years, 1.9 percent have been followed by miscarriage, Dr. Jackson has found. But he said this figure could not be compared with losses from amniocentesis because some of the women would have miscarried anyway.
Many obstetricians who offer the chorionic test simply tell their patients that the procedure has risks -uncertain but not enormous. ''We tell women that it is probably comparable in safety to amniocentesis, although it is certainly not less dangerous,'' Dr. Jackson said.
Dr. Rhoads and his colleagues are now conducting a study to determine how safe the chorionic test is. About 7,000 women at seven medical centers are participating - 6,000 have received the chorionic test and 1,000 have received amniocentesis. Within the next nine months, the investigators hope to have enough data to determine whether the chorionic test is as safe as amniocentesis.
In the second test, samples of the woman's blood are taken to measure levels of alpha-fetoprotein, a substance the fetus excretes into the amniotic fluid and that enters the mother's bloodstream.
The test was developed in England about 15 years ago, but it was slow to be adopted in this country, in part because a number of investigators and the F.D.A. feared it would be offered without adequate counseling and followup services such as sonograms and amniocentesis, if necessary. But about two years ago, the American College of Obstetricians and Gynecologists warned physicians that they might be subject to suit if they did not offer a woman the test and she later had a child with a defect.
The test was developed to detect the serious birth defects that occur when the neural tube does not completely close during early development of the fetus. In about half the cases, the tube is open at the top and the baby is born with a rudimentary brain, or no brain at all, a condition called anencephaly. These babies are dead at birth or die soon after.
In the other half of the cases, the opening is along the spine, and a portion of the nerve column of the spine is exposed. These babies, said to have spina bifida, are paralyzed below the portion of open spine. Some of these children also have hydrocephaly, a condition in which fluid accumulates in the head and that can result in brain damage. Children with spina bifida may be mentally retarded, and frequently have no bowel or bladder control.
Neural tube defects occur in about 1 out of every 1,000 babies born. In nearly all cases, there is nothing in a woman's medical history or family background to hint that she might be carrying a baby with a neural tube defect. The only way to find affected fetuses is with the protein screening.
When a fetus has a neural tube defect, large amounts of alpha-fetoprotein pour out of the open spine or skull into the amniotic fluid. From there they enter the mother's bloodstream, where they can be detected.
The blood protein test is given at 16 weeks of pregnancy, when there is enough of the protein around to make testing feasible. Those women whose alpha-fetoprotein levels are abnormally high are given sonograms, pictures of the fetus produced by sound waves, to see if there is some other explanation for the finding. If the sonogram reveals no explanation, the woman is given amniocentesis.
The amniotic fluid is checked for alpha-fetoprotein and for acetylcholinesterase, a nerve enzyme that is often present when the fetus has a neural tube defect.
In 1984, Dr. Irwin R. Merkatz of the Albert Einstein College of Medicine in the Bronx noticed that just as high levels of the fetal protein in a pregnant woman's blood indicate that her fetus may have a neural tube defect, so low levels may indicate that the fetus has a chromosomal defect, the most common of which is Down's syndrome. The finding was recently confirmed in a two-year study.
Dr. DiMaio and others throughout the country hail this result as an unexpected benefit of alpha-fetoprotein screening. Until then, woman were not normally offered amniocentesis unless they were at least 35 years old, the age at which the risk of carrying a child with Down's syndrome - 1 in 270 - is about the same as the risk that the procedure will result in loss of the fetus.
But, Dr. DiMaio notes, 80 percent of women who have babies with Down's syndrome are under 35; although their individual risks are low, most pregnant women are younger than 35.
With alpha-fetoprotein screening, it is possible to tell younger women if the fetus they are carrying is at increased risk of having Down's syndrome and to offer them amniocentesis. Not all cases of Down's syndrome will be detected by the blood protein test and most women who have amniocentesis on the basis of the test will not have fetuses with Down's syndrome, but the chance that they will is seen as high enough to warrant the risk of amniocentesis. Debate Over Blood Test
Still, the idea of offering blood protein tests to all pregnant women is far from universally accepted. Dr. Siegfried Pueschel of Rhode Island Hospital in Providence argues that Dr. DiMaio's study missed two-thirds of the affected babies and that if the test were used on all women under 35, more normal babies would be killed from amniocentesis than the number of Down's syndrome babies that would be detected.
But Dr. DiMaio and others take issue with this estimate because Dr. Pueschel cites a fetal loss rate of 1 percent to 1.5 percent with amniocentesis, whereas the usual estimate is one-half of 1 percent. Dr. Pueschel replied that the loss rate depends on where the procedure is performed.
Dr. Pueschel is also concerned that when a woman does have abnormal alpha-fetoprotein levels, she may be sent to a genetic counselor who may suggest that the best thing to do is to have an abortion. ''I have some problems with that,'' Dr. Pueschel said. ''Parents should be educated and informed. They should talk to the parents of a child with spina bifida or with Down's syndrome. If you talk to parents, you get a quite different opinion.''
The National Down Syndrome Society takes no stand on prenatal testing. The Spina Bifida Association ''supports responsible screening,'' meaning they are not opposed to the test in principle, according to Louise Schneiders of Walnut Creek, Calif., the group's secretary. But Ms. Schneiders said she is concerned that many women could choose to abort fetuses with neural tube defects because they have not been told ''the potential of a person with spina bifida.''
The alpha-fetoprotein test does offer one possible benefit that could reduce the amniocentesis rate. Women older than 35 with normal alpha-fetoprotein levels are at relatively low risk of having a fetus with Down's syndrome and so may elect not to have amniocentesis.
''The screening test is not perfect,'' Dr. DiMaio said. ''But it allows the detection of a birth defect that otherwise goes undetected.''
Some critics have wanted to slow the introduction of the new tests. But many doctors say there is never an ideal time to introduce new medical tests - there will always be nagging questions. And for many women, the benefits appear to outweigh the uncertainties, these doctors say.