A highly reassuring new report on the safety of Benedictin, the only medicine specifically approved by the Food and Drug Administration for use in early pregnancy, was made yesterday by a leading drug researcher.

The report concerns the alleged potential of the "morning sickness" drug to cause birth defects, particularly "reductions" in which part of a finger, toe, hand, foot, arm or leg is missing.

The unpublished evidence in the report shows the danger of congential limb anomalies from Bendectin use to be "very small or not present at all," Dr. Herschel Jick, the researcher, told a reporter. At the same time, however, he said, "only a fool" wouldn't advise women in the first three months of pregnancy to avoid any drug, if possible.

Jick, director of the respected Boston Collaborative Drug Surveillance Program, disclosed the data to a committee of FDA outside advisers. The panel, meeting at agency headquarters in Rockville, plans to decide today whether to recommend changes in the official prescribing instructions for Bendectin.

The drug is approved to combat nausea and vomiting in the first trimester of pregnancy. Experts emphasize the nausea often can be dealt with by eating dry crackers or toast and drinking very hot or very cold liquids.

The committee's action will have worldwide implications. Since 1956, Bendectin has been taken by or been available to an estimated 30 million women. U.S. physcians in 1978 wrote 3.4 million new and refill prescriptions for the medicine, sold by Richardson-Merrell Inc. In Canada and many other countries, the drug is sold over the counter as Debendox.

Other experts told the panel either that Bendectin has been shown to be a weak birth-defect agent, or that existing studies are inadequate either to incriminate or exornerte it. Some say too few women were involved in the Jick study.

The study was done in cooperation with the Group Health Cooperative of Puget Sound, Wash. It found six well-documented, unusual and relatively severe limb disorders in the babies of 2,966 Seattle-area women who used Bendectin, a birth-defect incidence of 0.2 percent. An equal number of such disorders occurred in the babies of 4,663 mothers not exposed to the drug, an incidence of 0.13 percent.

The relative risk -- 1.6 times higher in drug users than nonusers -- was not statistically significant, Jick said.

While the results "provide strong evidence that Bendectin is not a frequent cause" of serious limb disorders, they also "are consistent with a modest teratogenic [deforming] effect of this drug on the limbs." Jick told the panel. He also said the data provide no evidence that sustained use of Bendectin is riskier than infrequent use, pointing out that "none of the babies born of mothers who filled five or more prescriptions for Bendectin was affected."

Only four weeks ago, by contrast, a tentative version of the study showed a statistically significant risk among babies born to mothers who had filled two or more Bendectin prescriptions.

Jick, terming it "quite unfortunate" that the early version lacked, said the new report is based on 12 limb disorders -- three more than were known in August. In addition, he said, "a more detailed analysis of the extent of exposure" to Bendectin has been made over the last month.