Doctors at several prominent medical centers around the country -- and an unknown number of private physicians as well -- have begun helping some pregnant women abort their fetuses because the baby would be the "wrong" sex.

The number of these cases is now small, but some medical authorities predict -- and one bioethicist advocates -- a rapid increase in the use of sophisticated tests and abortions to allow parents to select the sex of their offspring. And the procedure is already raising important moral and ethical questions within the medical profession.

The issue centers around a prenatal test called amniocentesis, in which doctors withdraw a sample of the fluid in the womb. The test provides genetic information about the fetus, and is primarily used to detect birth defects. But it also reveals the baby's sex.

It is on the basis of this test that some patients choose abortion to avoid having an unwanted boy or girl, though some doctors say the preference in these cases is most often for a boy.

In most cases, authorities said yesterday, those doctors who agree to do amniocentesis only to determine a fetus' sex do so reluctantly. In the typical case, one or both parents are highly disturbed people who threaten to end the pregnancy in any case unless they can be sure the child is the desired sex.

Most doctors apparently still refuse to do an amniocentesis for sex determination alone even in cases like these.

But this situation could be changing.

Doctors at Johns Hopkins, Yale, the University of California at Los Angeles and George Washington University here say they will do such tests in a few, highly screened cases.

Dr. John C. Fletcher, a bioethicist at the federal government's National Institutes of Health in Bethesda, advocates that parents be permitted to use such tests for sex determination, if they choose.

Tabitha Powledge of the noted Hastings Center and Institute for Science, Ethics and Life Sciences at Hastings-on-Hudson, N.Y., said she thinks the practice is not yet really common, but could become so in two or three years.

Fletcher states his position in the New England Journal of Medicine. He said in an interview that he wrote as an individual, not as an NIH official, and came to his position "with great difficulty."

He finally decided, he said, that "it's a woman's right to decide her reproductive future . . . and if she's going to seek this procedure and abortion anyway, she should at least be able to get the very best medical care."

Doctors at Johns Hopkins' Prenatal Diagnostic Center have decided to "at least" counsel and advise parents who want the test for sex determination, and probably "reluctantly" agree in "one or two cases a year," reported Dr. Haig Kazazian, the center's director.

"Our position is that we will do it, in theory, in cases where the parents would abort the fetus if they did not know the sex," said Dr. John Larson at George Washington University here. "But this is a very restricted position, not an open-door policy."

The "actual situation", however, said the Hastings Center's Powledge, is that "more and more obstetricians are learning to do amniocenteses in their offices."

"As more and more learn," she said, "then in a way the lid will be off, because women will shop around until they find someone who's willing. And you can't tell me there won't be lots of obstetricians who are willing, for a price. I think the problem is probably not so much one of today as one of 1981 or 1982."

Dr. Maurice J. Mahoney, director of prenatal diagnosis at Yale University, said he believes "lots of obstetricians all over the country" -- some in private practice, some at universities -- are already quietly doing amniocenteses for sex determination.

"We're against it," he emphasized. "But under certain circumstances, we will get this information for a prospective mother, and we'll hope that this prevents" rather than cause "the sacrifice of normal kids."

He said Yale doctors have had "serious" requests for sex determination about twice a year, but only three women over the past 10 years have insisted on having the test after counseling about the risks and alternatives.

Neither of the two who learned that the fetus was not the desired sex chose abortion. "But we've given many other women information about sex," as well as genetic information, Mahoney added, "and we just don't know what they go on to do elsewhere."

He said "we would never agree to abort a pregnancy because of undesired sex," but pointed out that women can get abortions at many clinics today.

Dr. Barbara Crandall of UCLA recently told Ob. Gyn. News, a doctors' newspaper, that UCLA has done the test to determine sex in 10 cases since 1969, and, after counseling, none of the patients had an abortion. She added, "I don't feel I can deny this service to an emotionally stable individual with a reasonable case."

So far, said GW's Larson, his university has done the test only for sex determination, "when a patient tricked us into it by saying she wanted it for another reason.Then, when she found she had a child of an undesired sex, she asked her obstetrician to terminate the pregnancy. I don't know what happened."

"If we agreed to do the amniocentesis," said Johns Hopkins' Kazazian, "then our fertility control clinic would do the abortion. But we don't expect this to be very common. Our policy is to counsel such couples and tell them what it means and to a certain extent try to talk them out of it."

When an amniocentesis is done at a major medical center, a trained team takes a sonogram, or sound picture, to locate the fetus precisely, then inserts a fine needle into the abdomen to withdraw some amniotic fluid.

As done at Johns Hopkins, it costs $550. It can be done without hospitalization.

But most authorities think it should be done only at centers with skilled teams with much experience, since there is always some risk to the fetus. The risk, usually miscarriage, has been less than one in 100 at skilled American clinics.

In addition, however, the test can't be done until about halfway through pregnancy. This means that an abortion, if done, is a midterm operation with higher risk to the mother than an early abortion.

There are other dangers, including unknown consequences for society if parents widely seek this procedure, NIH's Fletcher writes. A doctor may state an opposing moral view, he argues, but should not withhold the test unless doing it would prevent its being done on some other child for more important reasons.