President Reagan said yesterday that the inability of anti-abortion groups to agree on the wording of a human life amendment was threatening to kill the legislation in Congress. In the same session with religious editors, he also maintained that babies have been born after only three months of pregnancy "and have lived, the record shows, to grow up and be normal."

Later, White House spokesman Larry Speakes acknowledged that the president had no proof of this statement. The best the White House could come up with was the example of 10-year-old Marcus Richardson, born prematurely at 4 1/2 months in Cincinnati in 1972, and now "a normal child."

Speakes said that the president's misstatement did not change his view or the point he was trying to make, which is that "as long as there is no scientific proof as to when life begins it is better to err on the side of saying that it begins at conception."

The president has frequently expressed the view that life begins at the moment of conception. And he has, throughout the 1980 campaign and his presidency, called for approval of a constitutional amendment that would prohibit abortion except to save the life of the mother.

Yesterday, however, Reagan for the first time expressed public reservations about the divisions within the anti-abortion movement, "because, first of all, they have not rallied behind a single measure."

"They've been divided behind . . . several amendments, and that's what kept us from bringing this to the floor and getting a determination," Reagan told the editors in a meeting in the Roosevelt Room of the White House.

Anti-abortion groups have had trouble all year choosing among a constitutional amendment and various bills that have been put forward on the subject.

The president, while expressing vigorous backing for the main anti-abortion legislation now pending, gave a bleak assessment of its prospects in the present session of Congress.

Reagan said that the pending measure sponsored by Sen. Jesse Helms (R-N.C.) had been "cluttered up with lot of extra and extraneous other resolutions which have weakened support for it and probably is one of the reasons that we are apparently not able to get closure to shut off the filibuster."

Reagan's pessimism was echoed on Capitol Hill, where abortion foes are considered to be about 10 votes short of the 60 required for cloture to end the filibuster against the Helms proposal.

Helms' amendment, attached to a "must pass" bill to raise the federal debt limit, would permanently prohibit the use of federal funds for abortions and abortion research or training. It includes two congressional findings, which do not have the force of law, one of which would declare that human life does begin at the moment of conception.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) told Republican colleagues yesterday that if a third attempt to end the filibuster fails today and a fourth fails Thursday he will move to strip the anti-abortion rider and another on school prayer from the debt limit bill.

Reagan made an emotional statement in behalf of anti-abortion legislation in his meeting with the religious editors.

"If you cannot determine when life begins, then doesn't simple morality dictate that you opt for the fact that it is alive until and unless someone can prove it dead?" he asked rhetorically.

"If we came upon a body in the street that was unconscious and we weren't sure whether it was unconscious or dead, we wouldn't say, 'Let's bury it,' " Reagan said. "We'd wait until someone assured us that it wasn't alive. And I think the same thing goes of the unborn child. I happen to believe the unborn child is a living human being.

"I think the fact that children have been prematurely born even down to the three-month stage and have lived to, the record shows, to grow up and be normal human beings, that ought to be enough for all of us."