The Supreme Court has been asked to rule on a requirement of all but one of the major airlines that a stewardess who learns she is pregnant must go on unpaid maternity leave immediately.

The exception, Northwest Airlines, allows a pregnant stewardess to work as long as she wishes, provided only after the sixth month she supplies a weekly doctor's approval of continued employment. In addition, Northwest provides special maternity uniforms.

The case before the court involves United Airlines, which until 1968 automatically fired stewardesses who become pregnant. Since then, under a union contract, United has required unpaid maternity leaves of at least seven months. Generally, a woman establishes that she is pregnant after the eight week.

United, the nation's largest airline, started the policy in the 1930s, citing an oral opinion by Dr. George Kidera, now its chief medical officer, that "medical reasons" justify it. Fatigue and morning sickness are among conditions that may occur in pregnancy.

The Federal Aviation Adminsitration, however, says that "a pregnant woman with no complications is in good health." Whether she should fly "is a matter between herself, her doctor and her employer," the FAA says.

In 1972, United ordered two suburban Virginia stewardesses who become pregnant, Karen Condit and Mary E. Oravec, to take unpaid maternity leave. As was customary, the airline did not have them examined to determined whether they were unable to do their work.

In behalf of themselves and about 1,000 other United stewardesses who had been similarly treated over a three-year period, Condit and Oravec then filed a class-action complaint in federal court in Alexandria.

The case centers on the Civil Rights Act of 1964, which allows an employer to take sex into account when it is a genuine or bona-fide occupational requirement.

Thus the key issue was whether United's requirement of non-pregnancy was illegal sex discrimination or a business necessity.

At the trial, medical experts divided on whether pregnancy might interfere with the primary safety mission of a stewardess: to help passengers out of an aircraft in an emergency.

They agreed only that "there are virtually no studies or empirical data directly relating to the issues here," Judge Oren R. Lewis Jr. found.

United's Dr. Kidera testified that Northwest's policy was to let pregnant stewardesses fly. "Do you know of any problems that Northwest had with that policy?" he was asked. "I never asked about them," he replied.

Did the physician know of "reports of pregnant stewardesses who could not to their work on any kind of flight because of fatigue or nausea?" Kidera's answer was, "No."

In September 1976, Lewis ruled for United. The 4th U.S. Circuit Court of Appeals upheld him last July. The stewardesses' petition for review will be acted upon by the Supreme Court after its recess ends Monday.

The lower courts held that pregnancy can incapacitate a stewardess in ways that could endanger safe operation of an aircraft, that United has an obligation to maintain the highest safety standards, and that the mandatory leave policy is "consistent" with that obligation.

Lewis did not rule specifically on whether the mandatory leaves were sex discrimination but said the policy "applies only to pregnant stewardesses. Clearly this is not a discrimination based solely on sex."