The mystique of the breast as a symbol of femininity and motherhood has almost always had its reflection in any age's standards of beauty. Despite the current obsession with thinness, "flat-chested" remains second only to "fat" as a term of derision.

To remedy this, more than 2 million American women are estimated to have had surgery to enlarge their breasts. The procedure, called augmentation mammoplasty, involves implanting silicone gel, encased in a half-moon shaped cover, under the skin of the breast. About 15 to 20 percent of implants are performed to reconstruct breasts removed in cancer surgery, while approximately 100,000 procedures are performed annually for cosmetic purposes.

Plastic surgeons have long maintained that implants are both necessary and safe. In a memo written in the early 1980s to the Food and Drug Administration, the American Society of Plastic and Reconstructive Surgeons wrote, "There is a substantial and enlarging body of medical information and opinion . . . to the effect that these deformities {small breasts} are really a disease . . ."

But women's groups, consumer organizations and a growing number of scientists, including some at the FDA, are questioning whether the improved quality of life touted by plastic surgeons is a reasonable trade-off for the possible health risks linked to implants: cancer; autoimmune disorders, such as rheumatoid arthritis; or lupus, a disease in which the body's connective tissue attacks itself.

Some question whether candidates for implants are fully aware of the risks. While the controversy has taken place largely out of public view, it was the subject last month of a congressional hearing sponsored by Rep. Ted Weiss (D-N.Y.).

Silicone implants were first marketed in the early 1960s. When the FDA began regulating medical devices in the mid-1970s, the implants were "grandfathered" in and manufacturers were never required to prove that the benefits outweighed risks.

The safety of implants has been questioned by some groups for several years, and the FDA has recently moved to regulate them after reports that they may break, may leak potentially dangerous silicone to other parts of the body or cause autoimmune problems, pain, infections and possibly even cancer.

Capsular contraction, in which the fibrous matter that forms around the implant becomes hard, lumpy and painful, is a common complaint, and affects 70 percent of women.

Since 1983, the FDA has received about 4,300 complaints concerning implants; about half of them are related to silicone implants and the rest concern those filled with a saline solution under a silicone cover.

Some scientists have suggested that leakage of silicone from implants may eventually cause cancer, perhaps in 20 or 30 years, in organs far distant from the breast.

Silicone and some of its breakdown products have produced cancers in animals, but there have not been enough epidemiological studies in women to determine whether cancer is a risk. Some radiologists say they are concerned that the presence of implants can prevent early detection of breast cancers by interfering with mammograms.

But many plastic surgeons say they believe that implants are safe and help both women who have undergone mastectomies as well as those seeking to enlarge their breasts.

The implant "is strictly a quality-of-life issue in terms of benefits," says Garry Brody, a plastic surgeon at the University of Southern California and a staunch advocate of silicone implants.

For the past 20 years, Brody has been following a group of 3,100 women who received implants. He says he has found no elevated risk of cancer or a failure to detect breast tumors among this group.

Other researchers have reached different conclusions. In a series of internal memos released by Weiss at the hearing, FDA scientists wrote that Brody's study did not "contribut{e} greatly to our understanding of the relationship between breast implants and the risk of breast cancer" because of "numerous sources of errors, biases and methodologic limitations."

Reviews of other studies led FDA scientists to conclude "that while there is no direct proof that silicone causes cancers in humans, there is considerable reason to suspect that it can do so."

One brand of implant is coated with foam made of polyurethane, which can release TDA, 2,4-toluene diamine, a known carcinogen. Some studies, including one performed by FDA scientists and now being replicated, found that TDA was released under conditions similar to those of an implant in a human breast. Here again, the evidence is skimpy, but scary.

Equally compelling is the concern that malignant breast tumors may go undetected. Weiss said that the American College of Radiology last year issued a consensus statement that "women contemplating augmentation be informed that mammography may be more difficult to perform and may be less effective." A 1988 study published in the Archives of General Surgery compared several hundred breast cancer patients who had undergone implants before they had cancer with those who had not. Researchers found more advanced disease, a higher percentage of invasive lesions and positive lymph nodes "resulting in a worsened prognosis." However there was no absolute proof that this was the result of implants.

The studies are inconclusive, but all of them underscore the need for further research. At the hearing, Weiss criticized the FDA for what he said appeared to be an unwarranted delay in making available scientific evidence it possessed about implants. He also chastised FDA officials for failing to heed the evidence presented by its own scientists about the risk of autoimmune disorders and potential cancer.

Brody, who called the hearings "one-sided," conceded that breast implants might make mammography difficult, but said that technicians could be trained to overcome this problem. He added that he had never heard of a delayed diagnosis of breast cancer resulting from an implant.

There is some evidence that silicone may produce a reaction that seems to cause symptoms of several autoimmune illnesses such as lupus.

Frank B. Vasey, a rheumatologist from the University of South Florida College of Medicine, testified that when he and his colleagues noticed that some of their patients had had implants, they began to recommend that the devices be removed. They reported on 12 such cases at a meeting last autumn of the American College of Rheumatology. After the implants were removed, 11 of the women recovered either fully or partially from autoimmune diseases.

Esther Rome, a member of the FDA panel that is reviewing the safety of implants and a co-author of the Boston Women's Health Collective book "The New Our Bodies, Ourselves," agrees that despite the risks, many women might still choose to have implants. "The question is," she said, "in our culture, is it okay for a woman to have small breasts or to have one breast?"

The problem is, she said, if a woman is allowed to accept an implant without knowing all the possible risks, then society is doing her a grave disservice. "Once again," she said, "a woman's looks are being considered more important than a woman's health."