THE ADS STARTED showing up in women's magazines in 1978, and you got the idea that there was something new and wonderful in the way of contraceptives -- something that was as effective as the pill and the IUD, yet something that did not have hormones and side effects.

What was new, in reality, was not much more than new packaging of the old spermicide that is in vaginal contraceptive foams. The new products were vaginal suppositories with names like Encare oval, Semicid and S'Positive, and while the ads didn't come straight out and tell you that they were 99 percent effective like the pill and the IUD, you could sure get that impression. S'Positive, for example, advertised itself as something that "looks like a pill. But it isn't. Yet, it does all the things you want the pill to do. And none of the things you do don't want it to do." You get the idea.

The suppositories were obviously the perfect contraceptive for women who did not want to use the pill because of its side effects. It seemed perfect, for example, for Ginger Zwolinski of Marquette, Mich., who was then the mother of two. "When the Encare oval came out, I just used that," she says. "They say they're as effective as the pill. They must be good. I relied on their advertisement, which was a mistake. I got pregnant with my third child, who is a year old now."

"I wrote a letter to Encare oval. I was very angry. I said I knew their product was the reason I got pregnant. I was pretty belligerent in the letter. I threatened to sue and make them pay for her. We only planned for two children. My oldest is 10 and the second is seven. That's all I wanted."

The only response she got was a brochure from the manufacturer in which someone had circled the operative small print. "They did not guarantee they were 100 percent effective. I realized there was nothing I could do," says Zwolinski. "I couldn't afford to sue them anyway."

Mary Hurley of Kalispell, Mont., at least got a letter back from the manufacturer when she complained. Hurley, who works in a health clinic and counsels teen-agers about birth control, figured she knew what she was doing. "Having read the advertisement, I was convinced it was a very effective means of contraception." She says she used the Encare oval, followed directions carefully, and "within three months I was pregnant. I had an abortion."

Hurley wrote the Federal Trade Commission to complain, and in her letter she pointed out that a lot of teen-agers she was counseling had been using the product because you can buy them without prescriptions and they are advertised as being extremely effective.

Hurley's complaint was one of more than 100 received by the Federal Trade Commission, which yesterday announced it has negotiated an agreement with the manufacturers of the three vaginal suppository contraceptives to tone down their advertising claims.

From now on, the manufacturers are going to have to state in their advertisements that their products are not as effective as the pill or the IUD and that they are about as effective as vaginal foams. Ads will have to include the information that users have to wait 10 or 15 minutes after insertion before having intercourse, that some women may experience irritation from the products, and that women who have been told by their doctors not to get pregnant should consult their doctors before selecting a contraceptive method.

All of this is well and good, but the fact of the matter is that it has taken the FTC a year-and-a-half to extract an agreement from the companies that they will not misrepresent the effectiveness of a contraceptive.

Tracy Westen, deputy director of the FTC's bureau of consumer affairs, says he doesn't know how many women became pregnant as a result of using the vaginal suppositories. Debbie Leff of the FTC says that manufacturers were doing between $6 million and $7 million a year worth of business in the products and that they accounted for 12 to 13 percent of the over-the-counter contraceptive market.

Westen says that given the medical and legal complexities of the case, the FTC moved relatively fast. The ads, he says, did not involve a clear-cut case of misrepresentation. The manufacturers did not claim, for example, that the products were 99 percent effective, like the pill. The ads were more subtle and more difficult to challenge.

Furthermore, Westen says, the products were new, at least in form, and medical information about their effectiveness had to be developed before a strong case could be brought against them in court for their advertising claims.

Maybe all that is correct. But surely there is something wrong with a system that allows misleading advertising for something as serious to consumers as contraceptives to be published and broadcast for more than 1 1/2 years without being challenged. No one knows how many women got pregnant and had to get abortions or bear unwanted children during that year-and-a-half, but Jeannine Michael, director of counseling at the Eastern Women's Center in New York City, did a survey that provides a clue.

Out of a sample of 30 women seeking abortions at the clinic, 22 had become pregnant while using Encare oval, the only contraceptive suppository she asked about. Eighteen of the women had heard of the product through advertisements, and most said they chose it because they believed it was effective and safe. The patients had used the product an average of a year.

The FTC got the manufacturers to change their advertising under the consent-agreement process, which allows companies to mend their ways without admitting they have done anything illegal. In fact, Norwich-Eaton, manufacturers of Encare oval, have issued a statement that says: "The company believes all prior promotions of its product, Encare, were correct. Encare continues to be a reliable and effective method of birth control that can be used with confidence by women."

Tell that to Ginger Zwolinski.