While the headlines have been going to budget cuts and El Salvador, anti-abortionists have without fanfare won several key positions in the Reagan administration and launched an effort to alter federal policy not just on abortion, but sex education, family planning and world population control.

The anti-abortionists have as their targets a long list of obscure and small but controversial federal programs that distribute contraceptives both here and abroad, teach teenagers how to prevent pregnancy, and provide a wide assortment of similar services.

Largely ignored in previous administrations, antiabortionists have found a friendly ear at the White House during the early weeks of this one. According to spokesmen, they personally asked the president in an early meeting to appoint one of their own as surgeon general and to revamp the world birth control programs administered by the Agency for International Development.

At a second meeting, Reagan and Office of Management and Budget Director David Stockman were given a "white paper" prepared by two anti-abortion groups saying that $3.9 billion a year could be saved by scrapping most of the government's family planning, birth control, sex education, teenage counseling and world population control programs.

Reagan subsequently chose Dr. C. Everett Koop, a leading anti-abortionist, as deputy assistant secretary of health and human services. Koop, a Philadelphia surgeon, is said to be in line to become surgeon general in a pending departmental reorganization. Among other things, this would put him in charge of programs administered by the Center for Disease Control, the Population Research Center, and the National Institutes of Health, all frequently attacked by anti-abortionists.

A fundamentalist Christian with a Lincolnesque board, Koop has been a board member of at least two anti-abortion groups, the National Right to Life Committee and Americans United for Life, and is the narrator of a controversial anti-abortion film, "Whatever Happened to the Human Race?"

A second anti-abortionist, Majory Mecklenburg, president of American Citizens Concerned for Life, is awaiting clearance to be named head of the Office of Adolescent Pregnancy Programs, which gives grants to aid teenage mothers and counsels them on birth control. Mecklenburg has said her organization believes teenagers should be taught to "postpone sexual involvement" rather than to use contraceptives.

Health and Human Services Secretary Richard Schweiker, who would supervise Koop and Mecklenburg, is a longtime supporter of a constitutional amendment prohibiting abortion and has said he would lobby for such an amendment in his present post. He also has said, "I don't think the federal government should be in the sex education business."

Congress has already moved in recent years to block federal funding of almost all abortions under Medicaid and other programs. Now those who favor choice on abortion and family planning experts are worried Congress and a sympathetic administration could go further.

Several pieces of legislation affecting family planning and world population control come up for renewal this year, and proponents are concerned that with the current emphasis on budget cutting these programs may become expendable. There are some reassuring signs. As a senator, Schweiker supported family planning programs, and in his confirmation hearings, W. Peter McPherson, the new AID administrator, said he generally supports population control efforts financed by his agency.

But in recent weeks there have been the following developments:

OMB Director Stockman has told at least one newsman he would like to eliminate population control programs financed by AID. Stockman also has proposed to combine a series of health, social service and family planning programs into block grants for states and cuts their funding by $2.5 billion, or 25 percent next year.

McPherson, in an interview, said he has assured Sen. Jesse A. Helms (R-N.C.), one of the Senatehs leading anti-abortionists, that he will "make sure" none of the agency's funds are used for abortions, and he will eliminate a $750,000 AID abortion research program. Part of this program has been aimed at finding ways to deal with botched abortions, a major health problem in Third World countries.

A potentially powerful antiabortion caucus has been formed on Capitol Hill with liberals and conservatives from both House and Senate joining forces. The caucus is headed by Helms and Thomas F. Eagleton (D-Mo.) in the Senate and Romano L. Mazzoli (D-Ky.) and Charles F. Dougherty (R-Pa.) in the House.

Anti-abortion legislators, led by Helms, Mazzoli and Rep. Henry J. Hyde (R-Ill.) have introduced a "human life bill," which would define life as beginning at the moment of conception and therefore make abortion murder. Karen Mulhauser, executive director of the National Abortion Rights Action League, argues this would make the use of several kinds of popular contraceptives the equivalent of murder, a charge anti-abortionists dismiss as a scare tactic.

Traditionally, anti-abortionists have been a fragmented minority in Congress. Now anti-abortion groups claim they can marshal from 200 to 250 votes in the House, and 41 in the Senate.

Two documents are noteworthy in the current attack for the way they expand the targets of the anti-abortion groups to birth control activities.

The first document is the "white paper" prepared by the Life Amendment Political Action Committee and the American Life Lobby. The groups say the document was given to Reagan and Stockman in a White House meeting Feb. 17.

Dealing primarily with domestic programs, it lists a series of specific grants for sex education activities and six major pieces of legislation mandating family planning activities. These are programs, the paper says, "that can be cut and at the same time do a great deal to advance the family and take the U.S. government out of the abortion and anti-family business."

The paper maintains "Planned Parenthood Federation of America receives untold amounts of funding through the federal government" from these programs and suggests some of that money goes for "abortion, abortion referral, contraceptives to minors, sex education, etc." Planned Parenthood representatives say this is untrue.

The second document is "Abortion and American Foreign Policy" authored by Patrick A. Trueman, executive director of Americans United for Life, a Chicago-based group. It is a wholesale attack on AID which Trueman claims is "foremost among the abortion promoting organizations in the U.S."

Since 1973, AID has been prohibited by law from using money to pay for abortions or "motivate or coerce any person to practice abortion." But AID is authorized to spend $190 million on programs to control population growth around the world this year.

Trueman argues these programs promote abortion by (a) financing abortion research (b) giving large grants for birth control activities to International Planned Parenthood, the Pathfinder Fund, and the U.N. Fund for Population Activities and (c) distributing IUDs and Depo-Provera, an injectable countraceptive that is effective for up to six months but is not allowed in this country because of potential health risks.

AID, according to spokesmen, does not distribute Depo-Povera, or any other contraceptive abroad that have not been approved for use in this country.

The three international agencies run about 60 percent of the birth control programs in developing world. The United States works through them, officials say, because of these agencies' long experience and expertise in population programs, their flexibility, their ability to work with private health organizations in developing countries, and the fact that many governments would prefer to work with them rather than directly with the U.S. government.

The stakes in the battle plan laid out in the two documents are exceedingly high. With population growing faster than food supply in many countries, about $1 billion is spent annually on birth control programs in developing countries. The United States is the largest single source of that money, supplying about 20 percent of it annually. Some 800 million women use the programs.

Domestically, the number of federally financed abortions has fallen from 295,000 to 4,430 a year since enactment of the 1976 Hyde amendment, which prohibited use of Medicaid for abortions except in cases of rape, incest and when the woman's health was threatened. (Another 204,000 abortions were paid for by states in 1979.)

Birth control, however, remains a major federal activity, providing contraceptive services and advice to 4.5 million women at 5,000 separate locations. According to Planned Parenthood, 42 percent of women receiving these services do so at local and state health departments, 27 percent from Planned Parenthood offices, 13 percent at hospitals and 18 percent from neighboorhood health clinics and community groups.

Basic family planning services are financed under Title X of the Public Services Act, which comes up for renewal this year. In 1981, they are budgeted to receive $166 million. Related activities, such as population research, teenage health care, sex education, and grants to states for year, are financed through at least three other major pieces of legislation. The budgets for these and other activities performed under these laws total over $173.5 million.