A method of postponing pregnancy which has no side effects, is virtually cost free, and can be 98 percent effective if used conscientiously is used by only 5 percent of American women. Why? This is the paradox of natural family planning.

By carefully observing her own body a woman can pinpoint the days of fertility and infertility in each cycle. What many women are unaware of is the great sense of freedom and power which understanding of this basic biology can bring. A woman who possesses this knowledge needs no more drugs or devices, no checkups other than her regular preventive care. For the rest of her life her fertility is under her own control.

A couple may avoid conception by abstaining from intercourse during the fertile time or enhance the chances of conception by timing their sexual contact to coincide with ovulation. It is the couple's choice.

Why do so few women use natural methods? Many have never heard of the ovulation method or the sympto-thermal method. They confuse natual family planning with calendar rhythm, an outdated and ineffective method of estimating a woman's fertile days from her previous cycles.

Promotion costs money, and there is obviously little profit to be made from natural methods. Instruction in natural family planning is individualized and ordinarily takes place over a perod of several months. Programs are available at a nominal cost at most Catholic hospitals and from several nonprofit organizations. After the method is learned it is cost free, in contrast with the millions of dollars spent on contraceptive technology every year.

Many women believe, or have been told, that natural methods are ineffective. Doctors sometimes give incorrect information out of ignorance. Only two out of four obstetrics and gynecology texts published in 1984 even contain information on the cervical mucus cycle upon which the newer methods are based. Changes in the cervical mucus signal the approach of ovulation.

Some doctors doubt whether women are intelligent enough to monitor their own fertility symptoms. Many dismiss abstinence as essentially unrealistic, especially from a male point of view.

However, the women they treat are the same women who spend much time and money to improve their health. Why assume these same women are unwilling to discipline the exercise of their sexuality if it means more freedom and better health?

Why is a very high failure rate for natural methods often cited in the popular press? Survey data contained in summaries of contraceptive effectiveness usually combine the pregnancy rates of calendar rhythm and effective natural methods. Furthermore, no distinction is made between method failure, errors of interpretation, and informed choice pregnancy: instead the three are added to compute the "failure" rate.

Dr. Hanna Klaus, a gynecologist who directs the Natural Family Planning Center of Washington, has been a researcher in natural family planning for over 10 years. She asserts that the method failure rate for modern natural methods is about 2 percent. Results from surveys all over the world have shown that natural family planning can be a 99 percent effective way of avoiding conception if competent instruction is obtained and indicators of fertility are carefully respected.

However, errors of interpretation are possible: natural methods should never be used without professional personalized instruction. Women who attempt to instruct themselves, who avoid what they vaguely assume to be "unsafe" times, or who assume their fertile days will be the same in all cycles often find that pregnancy results.

Most "surprise pregnancies" are actually no surprise at all. Natural Family Planning Physicians prefer to call these "informed choice pregnancies." If a woman chooses not to make systematic daily observations, or a couple observes signs of fertility but chooses to have sexual contact, pregnancy may well occur. This is not a failure of the method, only a failure to use the method. Research reveals that the pregnancy rate for couples who desire to simply postpone another pregnancy is much higher than the pregnancy rate of couples who intend to avoid another pregnancy.

Couples choose natural family planning for a variety of reasons. Natural methods have no harmful side effects and impose no health burden on the female partner. The pill is hazardous for women who smoke, are over 35, or have a number of predisposing conditions; the IUD can precipitate pelvic infections which may impair fertility or even endanger life.

The IUD is rejected by many couples on moral grounds. It functions not as a contraceptive but as an abortifacient: instead of preventing the union of egg and sperm, the IUD prevents the new life from implanting in the uterus. This mechanism sometimes fails. A pregnancy which does implant with the IUD in place must be carefully followed because of the danger of infection.

Most women are unaware that the pill is sometimes an abortifacient. Occasionally it fails to suppress ovulation. If ovulation and conception occur, other effects of the pill prevent implantation.

Foams, jellies, sponges and other barrier methods do prevent conception; however, all can cause irritation and in every case they interfere with sensation. Sterilization may be regretted later but is often impossible to undo.

Millions of dollars spent every year in this country on contraceptive drugs and devices could be spent in more satisfying ways. Millions of women, informed about their fertility, could be freed from the faults and foibles of contraceptive technology. Fertility is not a disease -- it is a natural part of being a woman or a man. Natural methods do require communication and cooperation, but isn't that what relationships are all about?