A federal appeals court in Richmond yesterday ruled that Eastern Air Lines can legally ground stewardesses after their 13th week of pregnancy and said not to do so amounted to "unreasonable experimentation" with passenger safety.

The ruling by a divided 4th U.S. Circuit Court of Appeals was immediately denounced by union leaders and feminist organizations. The Women's Legal Defense Fund called it "outrageous and ludicrous."

Four judges on the nine-member appellate panel also disagreed with the majority, saying that individual women react differently to pregnancy and that whether a pregnant stewardess can adequately perform her duties should be a medical, not a judicial or administrative, decision.

The ruling overturned an earlier finding by U.S. District Judge Robert R. Merhige Jr. that Eastern 'stewardesses could continue to fly between the 13th and 28the weeks of pregnancy with a doctor's written permission.

The airlines should decide whether stweardesses who are more than 13 weeks pregnant are capable of performing such tasks as opening emergency doors and windows, moving cumbersome lift rafts, carrying children and pushing passengers down emergency slides, the appeals court said.

By a 6-to-3 margin, the court agreed with Merhige that Eastern could not ground stewardesses during the early stages of pregnancy.

But the three dissenting judges said that even before the 13th week, "A stewardess who is vomiting in the lavatory cannot participate effectively in an emergency evacuation. Neither can one who is fainting or misscarrying." i

In a partial victory for the stewardesses, the court also joined Merhige in holding that Eastern may not downgrade pregnant flight attendents' seniority once they are transferred to ground positions.

"We now have to reconstruct what women would have transferred, going back to 1974 (when the original suit was filed), if they had had the assurance they would not lose seniority," said Robert B. Wallace, a District of Columbia attorney representing the employes.

Wallace said the amount of back pay for which Eastern would be liable could be substantial.

Bob Callahan, head of Transport Workers Union Local 553 in Miami, which represents the approximately 6,000 Eastern flight attendents, yesterday criticized the court's principal ruling:

"I challenge the airline to produce data on post-13th week hazards. Absent any data, then this is simply a marketing decision that fits some corporate idea of image and appearance."

An attorney for Eastern said yesterday the airline had not decided whether to appeal the decision.