The Supreme Court ruled unanimously yesterday that a woman cannot be deprived of her accumulated job seniority for taking maternity leave.

Such a deprivation imposes on women "a substantial burden that men need not suffer," Justice William H. Rehnquist wrote in the opinion for the court.

He pointed out that a loss of seniority can mean "less desirable and lower paying jobs" for the rest of an employee's career.

The court also held, however, that an employer can deny sick-leave pay for pregnancy, so long as the denial isn't a mere pretext for the sex discrimination outlawed by the Civil Rights Act of 1964.

Justice John Paul Stevens, in a separate opinion, wrote that his understanding of the law as it now stands is this: "Although some discrimination against pregnancy - as compared with other physical disabilities - is permissible, discrimination against pregnant or formerly pregnant employees is not."

The ruling came one day short of a year after the court upheld a General Electric Co. plan that provided equal benefits to men and women employees for disabilities other than pregnancy.

Sex-based discrimination "does not result simply because an employer's disability benefits plan is less than all-inclusive," Justice Rehnquist wrote for the court in the GE case.

In September, the Senate voted 75 to 11 to require employers to include pregnancy benefits in any workers' disability plans they offer. A House Education and Labor subcommittee has held hearings on a bill with a similar requirement and is expected to report it early in 1978.

Yesterday's decision involved the Nashville Gas Co., a Tennessee utility. It pays sick leave, without regard to sex, for disabilities, other than pregnancy, caused by accidents or afflictions unrelated to jobs. In this regard, its policy is shown by the record to be "legally indistinguishable" from GE's, Rehnquist said.

But there was another side to the policy, as Nora D. Satty learned when she left her clerical job in December, 1972, to bear the first of three children.

Returning three months later with her 3 1/2 years of seniority wiped out, she applied for three permanent jobs, any one of which would have been hers had she retained seniority. Each job went to a woman who had come to work after she did.

Nashville Gas did re-hire Mrs. Satty for a temporary project at $132.80 a week - an entry-level wage $10 under her former pay. She was fired when the project ended a month later. Her lawyer told a reporter yesterday she has been at home but wants to return to work at the company. He estimated the utility will owe her at least $30,000 in net back wages.

Rehnquist said that the Civil Rights Act did not allow Nashville Gas "to burden female employees in such a way as to deprive them of employment opportunities because of their different role."

Rehnquist noted that a federal judge saw no proof that business necessity dictated the utility's policy on loss of seniority and assumed that "no justification exists."

The judge was entited to so assume, Rehnquist said. Moreover, he wrote (the policy "might easily conflict with the [the company's] own economic and efficiency interests." For example, he said, the policy favors inexperienced over experienced employees and relatively new employees over others who "might be expected to be more loyal . . ."

In New York City, the American Civil Liberties Union's Women's Rights Project said the court had handed down "another confused decision on pregnancy" that says "discrimination against pregnant workers is sometimes illegal and sometimes not."

Justice Stevens also said that the legal distinction between permissible discrimination against against pregnant or formerly pregnant employees may case "some confusion."

Stevens, a dissenter in the GE case, offered a "rather pragmatic bases" for reconciling the situation:

In GE, the court upheld a disability plan that "did not attach any consequences to the condition of pregnancy that extended beyond the period of maternity leave."

In Nashville Gas, the court invalidated a seniority plan because it has "an adverse impact on the employee's status after pregnancy leave is terminated" and leaves the formerly pregnant person - always a female - "permanently disadvantaged as compared to the rest of the work force."

In a related action based on the GE and Nashville Gas decisions, the court nullified and sent back to a lower court a ruling that the Richmond, Calif., school board violated the civil rights law by refusing to pay accrued sick-leave pay to employees disabled by pregnancy and childbirth.