With jubilation tinged with only a small shadow of disappointment, women's health activists last week celebrated the surmounting of almost the last hurdle before U.S. sales of the cervical cap as an alternative contraceptive become a reality.

Last Wednesday, the Food and Drug Administration's advisory panel on obstetrical and gynecological devices gave a conditional recommendation to FDA Commissioner Frank Young that the Prentif Cavity-Rim cervical cap be approved.

The disappointment came when, after some eight hours of testimony, virtually unanimous in its support for cap approval, the panel decided to make its endorsement contingent on further testing.

At the same time, Victoria Leonard, director of the National Women's Health Network, which spearheaded the seven-year-long drive for cap approval, acknowledged that it was important to know if there was anything in the cap use that might account for cervical changes in the Pap smears of a few users in one study. ::

The cervical cap is a small, rubber device worn over the cervix, the opening of the uterus, to block sperm from entering. Like the more popular diaphragm, it is used in conjunction with a spermicide. Unlike the diaphragm, which sits in front of the cervix, the cap comes in direct contact with the cervix, fitting over it like a thimble.

Another difference between the two is that the cap is smaller and more difficult to put in place; the cap's main advantage is that it can be worn for several days at a time.

Dr. Gerald S. Bernstein, University of Southern California obstetrician-gynecologist, headed up the U.S. principal clinical trial of the cap, involving about 2,000 women at eight California clinics between 1982 and 1985. The women were fitted either with a cap or a diaphragm and returned to the clinic after one week and every three months thereafter.

Virtually everything showed that the cap and the diaphragm were comparable, but abnormalities on the Pap smears of fewer than half a dozen women have caused a ripple of concern.

The Pap smear, examination of cells scraped from the cervix, detects cellular changes that can precede cancer but also can result from inflammation or other temporary conditions. A sexually transmitted virus called papilloma is believed to contribute to cervical cancer in some cases.

Some of the diaphragm users also had abnormal Pap tests at the three-month mark, but more abnormalities were found among cap users. It was not a "statistically significant" difference, meaning the difference could have been the result of chance, an FDA statistician told the advisory panel, but it was "almost" so.

As a result, the company representing Lamberts Ltd., the British manufacturer of the cap, the women's health groups and the FDA are working out new tests that will determine if the abnormalities and the caps are linked. In the study, all the abnormalities were easily treated.

According to Bernstein, "the information is very vague at this point, but it does have to be looked at more closely. I think it is important to look at it and evaluate it more extensively.

"Still," he said, "it is not a major problem, nor is it reason enough to withhold the cap." Indeed, it is not anticipated that the April approval date would need to be delayed, as long as future tests are agreed upon.

Bernstein speculated that the abnormalities might have been caused by any number of things, many of which signified nothing. However, if there were a possibility that either the cap itself or the spermicide activated the papilloma virus or contributed to cervical changes in some other way, it would be important to know so that these women could be followed more carefully.

"That, of course, is why we do Pap smears," Bernstein noted. ::

Margaret Sanger, recognized as the high priestess of family planning, chose the diaphragm over the cervical cap early in this century probably because use of the cap -- or "French pessary," as it was known -- appeared to be more dependent on physicians. Her preference guided the diaphragm to its popularity and marked the almost complete disappearance of the cap in this country.

Victorian women in England "had their caps inserted by their gynecologists just after menstrual bleeding ceased, and then had it removed shortly before bleeding was expected to commence again," said Rebecca Chalker, who traced the history of the cap in her book "The Complete Cervical Cap Guide," published last year by Harper & Row.

The diaphragm was invented by a German anatomist in 1882, after the advent of vulcanization for processing rubber. Cervical caps, however, sometimes made of precious metals, such as gold, silver and copper, date back more than 2,500 years.

Nevertheless, although the diaphragm was "grandfathered" in, along with the condom, when the FDA began to require approval for medical devices in the 1970s, the cap was not included because it had not been widely enough used in this country.

When women's health activists determined that the cap could be less messy, less destructive of sexual spontaneity and just as effective as the diaphragm, they set about getting it approved. Initially, they simply wanted the FDA to waive its regulations but were persuaded by Dr. Lillian Yin that they should participate with the manufacturer in the proper legal methods of obtaining government approval.

Yin, chief of the FDA's Obstetrical-Gynecological Medical Devices Division, was at first regarded by the women's groups as a major opponent, but at meeting after meeting Yin convinced the women that following formal FDA approval procedures would benefit them all in the long run. A formal approval, she argued, would inspire more public confidence.

Yin worked out a compromise that permitted the issuance of about 100 "investigational device exemptions" under which some 50,000 women have been fitted with caps by health providers, including chiropractors and naturopaths as well as physicians, physician assistants, midwives and lay health workers.

Bernstein's study in California was underwritten by the National Institute of Child Health and Human Development and cost some $200,000.

The cap, once fitted, may be left in place for several days -- the FDA-approved label will suggest 48 hours, although women in most of the studies wore it between three and seven days. Its advocates believe that this feature, plus its size and bell shape, actually enhances sexual pleasure, a claim substantiated by answers to questionnaires in some studies.

However, it does have some disadvantages, which the studies brought out as well:Some women noticed an unpleasant odor. In Dr. Bernstein's study, this appeared to correlate with the length of time the cap was left on. Women who used it for three days or less were rarely troubled.Some women found it difficult to insert and remove. Most were able to master the technique after about a week, but, Bernstein told the panel, there were a few who simply could not. Fitters encouraged women to use a backup contraceptive method until the cap technique was learned.For unexplained reasons, the cap occasionally became dislodged in a few women.Some women were sensitive to the spermicidal cream or gel that is always used with the cap.Because the Prentif cap comes in only four sizes, there are some women who cannot be fitted. The health groups expect that once the cap is on the general market, the company will expand the size availability.

Advocates point out that none of these drawbacks is as serious as those that have been associated with some other contraceptive methods. Certain intrauterine devices, for example, have caused infections and deaths, although current designs are thought to be safe; some women, especially those who smoke or who are over 35, are not ideal candidates for birth control pills.

Meanwhile, other contraceptives are on the horizon. Bernstein is looking at some cervical caps that are disposable and have their spermicide built in. And a Wisconsin company has announced that it is seeking FDA approval for a "female condom."

This device, now known as the WPC-333, is made of polyurethane and is designed not only as a contraceptive but as an AIDS preventive. The device, which resembles a condom, is worn inside the vagina. Company spokesmen believe that they can obtain FDA approval quickly on the ground that the WPC-333 is equivalent to the male condom. (Regulations make provision for so-called "me-too" products -- deemed essentially identical to other already on the market -- to be approved without submission of all the data required of genuinely new devices.)

However, a source familiar with FDA procedures was skeptical that "a device designed to be worn by a women can be convincingly offered as a 'me-too' product when its equivalent is worn by a man."