In what U.S. officials described as a "major concession" to Third World countries at the U.N. Women's Conference, the United States today backed down on its insistence on rules that would have allowed it to block any final report.

The concession came on the opening day here of the 159-nation conference, which was threatened with immediate stalemate over the rules squabble. The meeting is intended to appraise the progress of women during the U.N. Women's Decade, which ends this year.

The U.S. delegation, headed by Maureen Reagan, the president's daughter, repeatedly has vowed not to allow political issues, such as Third World criticism of Israel and South Africa, to dominate the Nairobi conference, as they did previous meetings in Mexico City and Copenhagen.

To that end, the delegation had insisted for months that any final report of the conference be approved by a consensus rule that would have required agreement by every nation at the conference and thus given the United States veto power. With the rule, the United States had hoped to steer the gathering away from potentially embarrassing political resolutions and keep it focused, instead, on issues Washington considers more suitable for a women's conference.

In a series of closed-door meetings over the weekend and early this morning, however, the U.S. delegation was forced by Third World and East Bloc countries to abandon the consensus requirement.

In its place, the U.S. delegation agreed to compromise language that said the final report of the conference "should" be -- but does not have to be -- adopted by consensus. The compromise leaves Third World countries, which have an overwhelming majority here, free under conference rules to force into the final report any resolutions they wish. The United States will only be able to record one negative vote.

"If any group of countries wants to humiliate the United States, they are certainly free to do that. We are only one voice," Reagan said today in announcing that the United States had dropped its demand.

She insisted, however, that the compromise "was a moral victory" for the United States because it had forced Third World countries to make a "moral commitment to the idea of consensus." Reagan is a consultant on women's issues to the Republican National Committee.

Alan Lee Keyes, a U.S. representative to the United Nations with ambassadorial rank and the only male member of the U.S. delegation, said at a press conference today that the compromise would be a "moral and psychological 'restraint' upon those who would stubbornly insist that every last iota of their position has to be accepted and forced upon others." He said that when some delegations go back on the informal agreement, as appears likely from the preconference anti-U.S. rhetoric here, "they will bear the onus for introducing those division."

The retreat of the U.S. delegation to this new position, rather than opting to stalemate the conference over its demand for rule changes, appears to signal a desire by the United States not to derail the conference. Reagan said today that she has "no plans" to walk out of the conference, as was widely considered possible by many delegates.

The Reagan administration, in statements by the president and former U.N. ambassador Jeane J. Kirkpatrick, has made it clear that it does not intend to be pushed around at international meetings. And Maureen Reagan noted cryptically today that, despite the U.S. desire to compromise, "if somebody wants to push me to the wall, they will have to see what I am going to do."

Today's compromise was arrived at through negotiations led by U.N. Secretary General Javier Perez de Cuellar in a series of meetings with delegates from the United States, a group of developing countries known as the Group of 77 and the Soviet Union.