Scientists have developed a new fetal test to detect sickle-cell anemia, a crippling hereditary disease that chiefly afflicts blacks.

The test--cheaper, quicker and far more practical than previous methods--allows prospective parents to know if their child will have sickle cell. But it also could stir controversy. Scientists are increasingly able to detect genetic defects through prenatal testing, which raises the sensitive question of abortion. The new method uses amniocentesis, the withdrawal of a small amount of amniotic fluid from the womb for testing. The procedure has been successful in detecting Down's syndrome and many other genetic or physical defects in fetuses.

Doctors and scientists at the University of California at San Francisco, Harvard University and Johns Hopkins University have found a way to test fetal cells found in the amniotic fluid for sickle cell. In 19 such tests in the past two months, Johns Hopkins doctors have found eight fetuses with sickle-cell disease. In five cases so far, the parents have chosen to end the pregnancy by abortion. The parents in one case decided against abortion. The Johns Hopkins doctors have not heard the other parents' decisions.

In sickle-cell anemia, the red blood cells contain defective hemoglobin that causes the cell to become sickle-shaped when deprived of oxygen. Though its effects may be mild, it is often devastating, with recurring infections and painful "sickle crises." Many victims have a hard time attending school or holding a job, and many die early, though better treatment has reduced these ill effects in recent years.

One black in 10 in the United States carries the gene for sickle-cell trait or tendency. So do some Greeks, Arabs and other Mediterranean peoples.

Carrying the trait for sickle-cell is not the same as having the disease. But when two persons with the trait have a child, the chance is one in four that the child will have the disease. The disease is reckoned to afflict 40,000 to 50,000 American blacks.

Genetic counselors and doctors usually say they bend over backward to tell parents only "the options" and not recommend any particular choice.

"We can only tell them that sickle-cell disease is sometimes severe and sometimes very mild, and we have no way of predicting this," Corinne Boehm, a Johns Hopkins geneticist, said in an interview yesterday. "Our experience with previous tests is that about 50 percent of parents choose to terminate the pregnancy and 50 percent elect to carry it.

"We think the same figures will hold when we get enough of the new cases to be statistically reliable," she said.

Boehm said the availability of a practical test is likely to encourage more child-bearing than it halts. Geneticists say this frequently happens whenever a couple learn that a feared genetic disorder can probably be detected inside the womb.

The leader in developing fetal detection of sickle cell--and coauthor of one of two reports in today's New England Journal of Medicine--is Dr. Yuet Wai Kan of the University of California. Other authors are Judy Chang, Dr. Stuart Orkin, Dr. Peter F.R. Little and, at Johns Hopkins, Dr. Haig Kazazian and Boehm.

In 1975 Kan, with Harvard and Yale scientists, first diagnosed sickle disease in the womb. They did so then by fetoscopy, looking at the fetus through a slim telescope inserted in the mother's abdomen, then by taking a blood sample from a fetal blood vessel in the placenta.

This method proved dangerous, fatal to one fetus in 20.

In 1978 Kan started diagnosing the disease in stray fetal cells found in the amniotic fluid, the protective fluid around every fetus. A fluid sample is taken by amniocentesis, which also involves piercing the mother's abdomen, but at much less risk of harming the fetus.

Laboratory testing was cumbersome, however, requiring tests of the parents and other family members. Only the University of California and Johns Hopkins have regularly performed the 1978 test.

The new test only requires study of the fetus' telltale genes. It can be done in two weeks, rather than five. It is, said Chang and Kan, "safe and accurate."

Though the cost is not slight, it is much less than the previous method, which could easily run higher than $1,000. Hopkins will charge $300 for the test alone; the total with the amniocentesis, 80 percent of which Blue Cross usually covers, is $800.

Amniocentesis is now done routinely in major hospitals throughout the country. Private medical insurance typically covers a major portion of the cost, and Medicaid generally covers the procedure. But any new demand for hundreds of such procedures could swamp facilities.

In time, said the New England Journal authors, the laboratory work should become simpler and cheaper. Even now, said Chang and Kan, the method should be simple enough to start using it in Africa, where sickle disease is even more common than in the United States.

The new method borrows genetic engineering techniques to break up and examine DNA, or genetic material, in fetal cells. An editorial in the journal said such testing might reveal other genetic diseases in fetuses and become a way to diagnose certain viral diseases in sick adults.