Twenty years' use of the birth control pill has shown that early fears that it might cause major cancers were unwarranted, according to three major reports in today's Journal of the American Medical Association.

Two key federal health agencies--the Centers for Disease Control and the National Institute for Child Health and Human Development--have found no association between use of the pill and breast cancer.

They have found that the pill actually provides significant protection against ovarian and endometrial (womb lining) cancers. The endometrial cancer protection comes from "combination" pills, which include two hormones.

Also, Dr. Barbara Hulka of the University of North Carolina says in an editorial, recent reports call the risk of heart and blood vessel diseases very small for pill users under 35.

The pill still has its risks, however.

A study of Seattle women who used it for five to 10 years or longer found they had 2 1/2 to 3 1/2 times the normal risk of developing malignant melanoma, an often fatal skin cancer.

Dr. Elizabeth Holly, of the University of California at San Francisco, told a meeting of the American Society of Preventive Oncology in Bethesda yesterday that the risk was greater with larger-dose pills.

It is estimated that there will be 8,000 cases of malignant melanoma and 2,000 deaths from the disease this year among American women, whereas breast cancer strikes 114,000 and kills 37,000 women a year.

It is possible, authorities say, that future years will reveal associations between pill use and breast or other cancers, especially in vulnerable groups--for example, women with a strong family history of breast cancer.

"It may take 30 years" for breast cancer to develop, Dr. Sidney Wolfe of the Health Research Group said. "The pill may be useful for women who cannot or don't want to use other methods. But I certainly would not advise anyone to make it her first choice."

Scores of reports show that some pill users may have a wide range of adverse effects, including urinary and vaginal infections, gall bladder disease, depression and, sometimes, up to one or two years of infertility after quitting the pill.

Hulka, a highly regarded epidemiologist, writes:

"After 20 years . . . sufficient time has elapsed to evaluate the long-term adverse and beneficial effects . . . . There lingered a concern that the steroid hormone composition . . . would increase the frequency of cancer . . . . By 1980 women using the pill in 1960 through 1965 have had 15 to 20 years since their first exposure. This should be long enough for a cancer epidemic to be manifest."

With all the latest information, she concludes: "New assessment of the risks and benefits can be made . . . . For now, the long-term news is good."

Government scientists studied the cancer histories of women aged 20 to 54 at eight regional registries. They compared 689 cases of women who have breast cancer with 1,077 "controls"--healthy women--and found no increased risk of breast cancer in pill users.

The scientists also compared 170 women with ovarian cancer with 1,642 controls; pill users had 40 percent fewer cases of ovarian cancer than non-users.

Comparing 187 women with endometrial cancer with 1,320 controls, they found that combination pill users had half as many endometrial cancers as non-users. Past and present pill use is preventing 3,700 cases a year of ovarian and endometrial cancers, the scientists estimated.

Other recent reports have stressed that the pill's main risk is cardiovascular (heart and blood vessel) disease--thromboembolism (blood clots), heart disease, stroke or high blood pressure--with most of the risk concentrated in women over 35 and in pill users who also smoke.

A respected 1981 British report showed that among pill users under 35, the excess risk of death from cardiovascular disease was 1 per 77,000 per year for nonsmokers and 1 per 10,000 for smokers.

For women aged 35 to 44, the risk was 1 per 6,700 women for nonsmokers and 1 per 2,000 for smokers; for women over 45, 1 in 2,500 per year for nonsmokers, 1 in 500 for smokers.