The Supreme Court let stand yesterday a ruling that United Airlines does not engage in illegal sex discrimination by requiring stewardesses to take unpaid maternity leave when they learn they are pregnant.

All major airlines follow the same policy except Northwest, which requires only that a working pregnant stewardess supply a doctor's approval weekly after the sixth month.

The ruling, made by the 4th U.S. Circuit Court of Appeals, applies in that circuit's jurisdiction: Maryland, Virginia, the Carolinas and West Virginia.

The unsuccessful challenge to United's policy was brought by two former stewardesses, Karen Condit and Mary E. Oravec of suburban Virginia.

They attacked the requirement as illegal sex discrimination under the Civil Rights Act of 1964.

United said the requirement was a business necessity under the law, which allows sex to be taken into account when it is a genuine occupational requirement.

The appeals court ruled that the requirement is "consistent" with United's obligation to maintain the highest safety standards, stewardesses being responsible for getting passengers out of an aircraft in an emergency.

In petitioning for Supreme Court review, Condit and Oravec said that by holding that an employer's policy need merely be consistent with sex exemptions in the law, the 4th Circuit was enabling employers to subject women "to discriminatory policies in every area of employment where safety conceivably could be at issue.

In a related action, the court let stand a 7th U.S. Circuit Court of Appeals decision upholding a "no-spouse rule."

Under the rule, imposed by Libbey-Owens-Ford Co. at its Ottawa, Ill., plants about 25 years ago, an hourly employe will not be hired if a spouse is doing similar work.

The rule was neutral on its face but excluded women 24 times as often as men. A trial judge held that the rule violated the civil rights law. The appeals court reversed him and, yesterday, prevailed.

Two of the nine justices, William J. Brennan Jr. and Potter Stewart, wanted to hear arguments in the case.