Even if women used the two most effective available methods of contraception more regularly, the need for abortions in the United States would not be eliminated, a new family planning study has found.

Research sponsored by the Alan Guttmacher Institute concluded that if all women used the pill or the intrauterine device, a minimum of 450,000 abortions would be performed annually in the United States, or 30 percent of those now performed each year.

Princeton professor Charles F. Westoff, who was in charge of the study, believes this amounts to the "irreducible minimum" in abortions, but added that it is an "extreme" that is neither likely nor necessarily desirable, given the continuing debate over the side effects associated with these birth control methods.

If more women could be persuaded to use any of the assortment of methods available, he said, about half of the abortions each year "could be prevented, but the other half could not."

Even if the rhythm method--one of the least effective methods--were used by women not now using contraception, abortions could be reduced by 44 percent, the study suggests.

Wendy Baldwin, an official at the federal Center for Population Research, called the study an "extremely important contribution" to the policy debates over abortion "because people can weigh alternatives in a more informed way."

The calculations are based on data collected during 1980 from more than 4,000 abortion patients at 14 major facilities in Illinois. Westoff said they are considered the first estimates of their kind.

The study was funded by the Ford Foundation under auspices of the Guttmacher Institute, an independent research agency affiliated with Planned Parenthood which favors the continued availability of legal abortion.

The figures indicate that more could be done to encourage women to use contraception. About 60 percent of abortion patients reported they did not use any method of birth control at the time they conceived.

In addition to reaching women--especially teen-agers--with birth control information before they initiate sexual activity, the authors said there is a need for more abortion clinics to offer "contraceptive advice and services following the abortion, when women are apparently highly motivated to prevent another unintended pregnancy."

Westoff said information gathered for the study indicated that about half of the clinics studied offered no such post-abortion advice on contraception.

He and his colleagues noted that, even if family planning were more universally practiced, limitations in available contraceptive methods would continue to produce large numbers of unwanted pregnancies.

"Many women obtaining abortions became pregnant in spite of their efforts to prevent conception, either because they did not use the method correctly or consistently, or because, in spite of correct use, the method was not effective at preventing pregnancy," they wrote in the current issue of Family Planning Perspectives.