The number of mentally retarded persons could be cut in half if parents would take proper care of their children before and after birth, Rosalynn Carter said here today.

She cited a scientific report to back up this statement in an address to the National Association of Retarded Citizen's convention, but, she said, "We need more research and we also need to apply this knowledge that we have where it is needed."

Such research would have to be done by private and public agencies, she said, and the results of such work should be given the widest possible coverage to help other people.

As an example of such coverage, she cited a series of stories by Robert Meyers in The Washington Post on his mentally retarded brother, Roger, who had been able to find help and, despite his handicap eventually move into an apartment of his own and marry a mentally retarded woman he had met in school.

"I hope if you have not read that story you will read it," she said. "Roger's life proves this: we must widen our definition of what is normal in society. As he himself says, 'People are retarded by what they don't know. That's why I work so hard to learn the things I don't know and to become more normal.'"

Robert Meyers had flown in for the speech, and the First Lady asked him to stand. He did - to sustained applause from the approximately 2,700 convention delegates.

In a news conference after her speech, Mrs. Carter said she favors an extention of the deadline for ratifying the Equal Rights Amendment, and she said she does not think President Carter will leave on his extensive trip, planned to begin Nov. 22 until an energy bill that he likes is on his desk for his signature.