More than half of the pregnancies in the United States are unintended and nearly half of these end in abortion, an independent research group said yesterday in a study of the use and risks of contraceptives.

The Alan Guttmacher Institute said that unplanned pregnancies are a greater threat to women's health and lives than any contraceptive.

"Fear and confusion" cause many women, especially the young, to shun contraceptives or use them sporadically, according to the study conducted by Guttmacher, which is affiliated with Planned Parenthood.

"The headlines on the pill's harmful effects have made many teen- agers stop using them," said Dr. Howard W. Ory, senior author of the study and deputy director of epidemiology programs at the federal Centers for Disease Control in Atlanta.

"Women under 25 get maximum benefits from the pill and are at minimum risk, but they're the ones stopping its use."

If health were the only consideration, the study said, "a woman's safest course would be to start with the pill . . . for four years to get the maximum protection against ovarian cancer, have the number of children she desires by the mid-20s and then persuade her husband to have a vasectomy."

Studies show that the pill reduces the risk of both ovarian and endometrial cancer, or cancer of the lining of the uterus.

Studies also suggest the pill may reduce the risk of developing benign breast tumors. Many doctors consider it less dangerous for a man to have a vasectomy than for a woman to be surgically sterilized.

The institute said:

Women suffer greater risks from contraception than from childbirth only if they take the pill when they are over 40, or use the pill and smoke when they are older than 35.

Some 500 of the 10 million pill-users die each year from causes related to pill use, mainly strokes and heart attacks, compared with close to 30 from complications of tubal sterilization, 30 from the interuterine device IUD and l5 from abortion complications.

The pill "prevents more illness than it causes." The 500 deaths attributed to pill use are offset statistically by an estimated 850 ovarian cancer deaths prevented by pill use.

The diaphragm and condom, while relatively complication-free, may not be as effective, on the average, as has been thought. Almost 10 percent of women who rely on their husband's use of condoms in the first year of marriage get pregnant. More than 18 percent of women who rely on diaphragms during the first year of marriage get pregnant.

Other failure rates that prevail during the first year of marriage: vasectomy or tubal sterilization, 0.4 percent; the pill, 2.4; IUD, 4.6; spermicides, 17.9; rhythm, 23.7.

Complications each year hospitalize 0.4 percent of IUD users, mainly by causing or spreading pelvic inflammatory disease; 4.5 percent of those who have tubal sterilizations and 0.3 percent of those who have legal abortions. About 0.l percent of pill users have major complications each year.

Overall, 56,000 or 0.3 percent of the 14.6 million women who use the riskiest methods--the pill, IUD, tubal sterilization and abortion--suffer major complications each year. By comparison, 60,000 or 2 percent of the 3 million sexually active women who use no birth control method have either Caesarean sections or ectopic pregnancies, "only two of the many major complications of pregnancy and birth."

The institute compiled these statistics from surveys by the Centers for Disease Control and various universities.

Dr. Sidney Wolfe, director of the Public Citizen Health Research Group, said yesterday the pill still has not been used long enough to know whether it will cause some breast cancers, a possibility Ory called unlikely.

Carefully used, Wolfe said, "The diaphragm is very effective. I wouldn't recommend the pill as first choice."