A leading population expert Louis Hellman of the respected Population Reference Bureau, estimated yesterday taht, under a Senate-passed restriction on federal financing of abortions, 90 per cent of those now performed could still be done.

That is because of a number of exceptions written into the Senate legislation, and especially a very broad exception permitting any abortion "medically necessary," Hellman told a reporter.

A leader of the anti-abortion government in Congress, Rep. Henry J. Hyde (R-Ill.), made a similar observation. He called the Senate version of the legislation "a Christmas tree of exemptions and loopholes. It permits abortions for everything, including athlete's foot."

These estimates mean the abortion fight in Congress may not be quite over. There is certain to be a bitter battle in conference between the Senate and the more restrictive House when Congress reconvenes next week.

The anti-abortion provisions are riders on the Labor-Health, Education and Welfare appropriations bill.

Hellman, former deputy assistant HEW secretary for population affairs, said in response to a question that "my guess and it's just a guess," is that if the Senate language prevails in the conference, it would reduce the number of federal funded abortions by only about 10 per cent, leaving 90 per cent still possible.

On the other hand, said Hellman, if Congress eventually adopts the tighter restriction favored by House conferees, which would permit federal funding of abortions only to save the life of the mother, federally financed abortions would drop from about 300,000 a year under the federal-state Medicaid programs to "only a few thousand - possibly 1,600 a year" or less.

All these figures are purely an estimate," stressed Hellman, who was a leading figure in the Medicaid abortion program before leaving HEW.

The dispute about the proposed curb on federal funding of abortions will probably be the most impassioned and difficult issue when House-Senate conferees meet soon after the recess ends next week to resolve differences between the two chambers' versions of the multi billion-dollar funding bill for the Labor Department and HEW.

The House bill in its present form bars all federal funding of abortions. But Hyde said he (and he believes most of the House conferees, headed by Democrat Daniel Flood of Pennyslvania) want to add a single exemption to that, permitting federal financing when necessary to save the life of the mother. This is the administration's preference as well.

The additional Senate exemptions were put in by Sen. Edward W. Brooke (R-Mass.) on the floor by a 56-to-39 vote.

The Brooke amendment would allow federal funding of abortions to save the mother, or for the treatment of pregnancy caused by rape or incest or where "medically necessary."

"Medically necessary" is the phrase that Hyde and others believe will open the floodgates, since in practice it can include situations ranging from severe and immediate danger to the life or health of the mother, to the danger of birth of a deformed foetus, or a threat to the mental stability, health or well-being of the mother. A woman couldn't just have an abortion if she felt like it, but the doctor would have very wide discretion.

"Medical necessity is as about as subjective as you can devise," charged Hyde. (In California, officials said in a phone interview, physicians list to 90 per cent of Medicaid abortions as "therapeutic.")

Anti-abortion groups are opposing the Brooke amendment. However, a group of women's organizations and medical groups are backing it. They say the only fair way to decide whether to fund an abortion is allowing a woman and her physician complete discretion to decide, on the basis of all factors in the woman's situation, including her physical health, mental health, family situation, and whether she had fetus-endangering illnesses during pregnancy. These organizations include the National Abortion Rights League, American Civil Liberties Union, a group of religious organizations. National Organization for Women, Planned Parenthood and several others, according to Karen Mulhauser of the league.

The American College of Obstetricians and Gynecologists over the years has adopted yardsticks for guiding its members on when to perform abortions. They include situations where the life or mental or physical health of the woman would be seriously impaired, rape or incest, the likehood that the child will have grave physical deformities or mental retardation, or the need to "improve the family life situation."