The Supreme Court was told yesterday that New York City faces financial ruin if it must reimburse women employees who were unconstitutionally forced to leave their jobs because of pregnancy.

L. Kevin Sheridan, an assistant New York City corporation counsel urged the court to deny back pay to women who were forced to quit their jobs because of pregnancy before it was medically necessary.

"This could open up litigation that would be ruinously expensive" to New York and other financially costs might run to tens of millons of dollars.

The case arose in 171 after Jane Monell, a welfare caseworker in New York, and a number of city schoolteachers were forced to take maternity leave in their seventh month of pregnancy. Their doctors advised them there could be no harm medically in working another month.

The city changed the policy the following year to permit women to work as long as they could. The policy change was made after a suit was filed challenging the constitutionality of the city's maternity leave rule.

In 1974, the Supreme Court declared that policies like the one New York had rewaked unconstitutionally discriminated against women.

But the issue of whether the women are entitled to back pay for the period of work they were forced to miss has not been determined.