The first direct evidence linking intrauterine devices to infertility was disclosed yesterday, prompting medical scientists to warn that young childless women generally should avoid using IUDs if they wish to have children in the future.

Two major federally sponsored studies suggested that at least 88,000 American women may be unable to have children because of reproductive damage following use of that form of birth control.

The findings were not all negative. They also indicated that risk of infertility from IUDs varies widely from one type to another and that copper-wrapped IUDs may present no increased risk of infertility for women who have had a child, who are over age 25 when they first begin using the device or who have had only one sex partner.

While there is agreement that one IUD -- the discontinued Dalkon Shield -- should be removed, the scientists said yesterday that women should consult with their physicians about the advisability of removing other types of IUDs.

The IUD -- a small plastic or copper-wrapped plastic device implanted in the uterus to prevent pregnancy -- is the fourth most common type of contraception in the United States, behind voluntary sterilization, birth control pills and condoms.

The National Center for Health Statistics estimates that about 8.6 million American women have used an IUD, including 2.2 million current users. A spokesman said about 1.1 million childless women have used an IUD, including about 275,000 current users.

The new studies found that women who have not yet given birth are twice as likely to become infertile if they have used IUDs as women who have never used them.

"If [women] wish to have children in the future, they should use some other method," said Dr. Janet R. Daling, a University of Washington researcher who headed a study of 318 women in the Seattle area.

Dr. Daniel W. Cramer of Brigham and Women's Hospital in Boston, who headed a national study involving 4,185 women, agreed, saying IUD use should not be a birth control method of "first choice" for women who have not have children.

But the researchers -- as well as officials with the National Institute of Child Health and Human Development, which funded the new studies, stressed that the increased risk of infertility varied widely -- depending on the type of IUD used.

Plastic IUDs, particularly the Dalkon Shield, carried the highest risk, while the copper-wrapped IUDs in wide use were the safest, with only a modest elevation in risk.

An editorial in today's New England Journal of Medicine, in which the new studies appeared, added that IUDs generally are "particularly useful for women over the age of 30 years. In this age group, the risk associated with using oral contraceptives and the effectiveness of the IUD are both increased." The author, Dr. Daniel R. Mishell Jr., noted that IUDs are among the most effective, reversible forms of contraception, with pregnancy rates of 1 to 5 percent in the first year of use and declining thereafter.

The research is likely to have a major impact not only on contraceptive choices but on legal battles involving women who claim they are infertile because of IUD use.

"Both of these investigations confirm what doctors have strongly suspected for years -- that the IUD increases the risk of infertility," said the institute's Dr. Bruce Stadel. "We've known since the mid-1970s that the IUD raises the risk of pelvic infections, which can impair fertility. But until now we haven't known just how often different types of IUDs lead to infertility."

Insertion of an IUD tends to be more difficult in women who have not given birth, resulting in complications such as bleeding, pain and expulsion of the device. There is also a greater rate of pelvic infection in childless women using IUDs.

The fertility problems linked with IUDs involve "tubal infertility," in which the fallopian tubes are damaged, preventing the woman's egg from traveling from ovary to uterus. Researchers have long believed that IUD use increases the incidence of pelvic inflammatory disease because the device's "tail," which leads out of the uterus into the vagina to facilitate removal, may provide a means for bacteria to move into the reproductive organs.

The Boston-based study, conducted at seven centers including George Washington University, found that overall the use of an IUD in the past among childless women doubled their risk of tubal infertility. Risk was greatest among short-term users with complications such as infection or pain and among users of more than three years.

The Seattle study found that the risk of tubal infertility was 2.6 times as great for IUD users. Even women who have had no symptoms of pelvic infection with the IUD have a greater chance of becoming infertile with the IUD, it found.

The incidence of tubal infertility in the 54 million American women of reproductive age (15 to 44), both users and nonusers of contraceptives, is roughly 2 percent.

The two studies found that the risk among childless women who had used a Copper-7 or Copper-T IUD was 1.3 to 1.9 times greater than among non-IUD users. The risk was about three times greater in users of the Lippes loop or Saf-T-Coil devices, and the highest risk -- 3 to 7 times greater -- was among Dalkon Shield users. The Saf-T-Coil and the Dalkon Shield are no longer marketed.

While estimates vary, Cramer said about 10 percent of American couples of reproductive age -- about 2.75 million couples -- have trouble conceiving a child. Of them, about 20 percent, or 550,000, are thought to have tubal infertility. The infertility of about 16 percent of those women -- about 88,000 -- is now linked to IUD use.