Every year some 5 million women get help from federally supported family planning clinics in the United States. Millions more receive family planning services in developing countries through programs supported in part by U.S. funds.

Now vast numbers of those women -- at home and abroad -- are in danger of being cut off. Frustrated by the courts, the Congress and public opinion, and urged on by single-issue ideologues, the Reagan administration is waging all-out war against abortion, and in so doing, is weakening family planning efforts.

Here in the United States, about 85 percent of those who use government-supported family planning clinics are poor; nearly one-third are adolescents. Some go seeking information about birth control; some are screened for sexually transmitted diseases, some get nutrition and fertility counseling. Most are there because they think they may be pregnant, and they want to be tested.

If it is determined that a client is pregnant, the usual practice is to ask her how she feels about it. If she says, ''Wonderful,'' then the clinic staff will offer its congratulations and tell her where she can go for prenatal care, so vital to the health of mother and baby.

If she's not happy about being pregnant or not certain how she feels, then a counselor will tell her what her options are -- to have the baby and keep it or put it up for adoption, or to terminate the pregnancy. If she decides on an abortion, she'll get information about where she can go for it.

But under the administration's new ''gag rules,'' imposed without consulting Congress, clinics receiving federal money will no longer be able to say anything about abortions -- not even if a woman has a medical condition that might require an abortion to protect her life or health, or if she requests information. If the ''A'' word is so much as mentioned, the clinic stands to lose its federal support. In fact, says a critic of the rules, it's not clear whether clinics will still be able to tell pregnant women where to get prenatal care.

The Reagan regulations are ill-conceived and misguided.

In the first place, they are unfair. Clients of these family planning programs overwhelmingly are poor women and teen-agers who cannot afford a private physician. For most, the clinics are their entry point into the health care system and the only source of easily available health care.

Second, the policy is economically unwise. Every dollar invested in family planning saves two or three dollars in future health and social service costs. We have about 1 million teen pregnancies a year in this country, 400,000 of which end in abortions. Putting limits on family planning clinics is sure to increase the number of unwanted pregnancies and welfare children.

There also is concern that the rules are designed to shift the emphasis of family planning services from furnishing health services to providing babies for adoptions. In the section of his State of the Union address attacking abortion, President Reagan pledged to ''remove barriers to adoption.''

These new regulations really shouldn't come as a surprise. The administration did the same thing to U.S.-backed family planning services overseas. In 1984, without hearings or congressional approval, it changed U.S. policy to cut off funds for any agency that supports nongovernmental foreign organizations engaged in abortion-related activities. The cutoff applies even if such activity is legal in the foreign country and is paid for with locally raised, non-U.S.-government money.

Ronald Reagan and his anti-abortion allies long have been trying to cut back or alter family planning services. The public has said, ''No.'' Congress has refused to slash funds. The courts have ruled that Americans have the right to determine if and when they want to have children, and that women have a right to choose whether or not to have an abortion.

Now the administration is thumbing its nose at all these elements of society and stepping up its ideological policy-by-fiat.

Unless Congress shows more spunk than it has so far in standing up to the administration on family planning, it will be up to the courts to halt this unwise and unfair policy.

Otherwise, here at home, counseling will be biased and incomplete, and a vital part of America's public health program will be hopelessly crippled, or killed. And overseas, many thousands of families in the world's poorest nations will be burdened with unwanted children and will sink deeper into poverty and suffering.