The Food and Drug Administration yesterday gave conditional approval to a prenatal test kit designed to screen for spina bifida and other major birth defects.

The agency told companies planning to market the inexpensive blood-test kits that they could begin selling the product as soon as labeling and patient information brochures were approved.

But the nonprofit Health Research Group called the FDA action a "grave mistake" and said it would challenge the agency's decision in court. Health Research Group director Sidney M. Wolfe said that approval of the test without stringent restrictions on its use "could lead parents to abort healthy and wanted children they mistakenly suspect to have a neural tube defect."

Between 3,000 and 6,000 of the 3 million children born in the United States each year are afflicted with neural tube defects, which result from malformations of the spinal chord and brain stem in the fetus and include spina bifida, which results in varying degrees of disability.

The test, given between the 16th and 18th weeks of pregnancy, measures the level in the blood of a substance that rises to abnormal concentrations when neural tube defects are present. But the the substance, called alpha-fetoprotein (AFP), is also elevated in the blood of twins or of fetuses older than 20 weeks.

Although a negative result in most cases would confirm the absence of neural tube defects, a positive test would not necessarily indicate a deformity. Positive results should be followed by such prenatal diagnostic procedures as ultrasound and amniocentesis, the FDA said.

"The AFP test is valuable only as a first step," Wolfe said. "The problem is in the many false positives one can expect if the tests are widely marketed." Careful monitoring and counseling are essential to prevent unnecessary abortions, he said.

In yesterday's action, the FDA dropped restrictions proposed in 1980 that were meant to reduce the possibility that the test would be misinterpreted. The restrictions, which would have required extensive record-keeping, physician education and laboratory quality control, were criticized by the American Medical Association as overly restrictive and by manufacturers as too expensive, the FDA said. In dropping the proposed restrictions, an FDA spokesman said, the agency wanted "to keep the government out of the doctor-patient relationship."

The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists, in a joint statement, criticized the FDA's decision to drop stringent marketing restrictions on the test kits. The groups questioned "the appropriateness and effectiveness of this screening test for all pregnant women."