Even though it is a popular method of birth control around the world, the intrauterine device, or IUD, fell out of favor in the United States about 20 years ago. Everyone feared that it caused infertility in young women who had never been pregnant. However, a study published this week concludes that it was not the IUD that made them infertile, but a sexually transmitted disease.

IUDs covered with a small amount of copper are by far the most common. To find out whether they made American women infertile, researcher David Hubacher traveled colleagues South of the border. "We conducted this research in Mexico City because Mexico is a country where everywhere IUD use is well accepted," he said. "And there are many women who are using the method there."

Investigators from Family Health International in Research Triangle Park, North Carolina, along with some Mexican colleagues, recruited nearly 2,000 women at three hospitals.

Mr. Hubacher, an epidemiologist, says the women who had previously used copper IUDs had what is known as tubal infertility. That meant their fallopian tubes were so badly scarred, an egg could not enter the uterus from their ovaries to be fertilized by sperm. The other infertile women could not get pregnant for undetermined reasons.

Mr. Hubacher says researchers also took blood samples from the women so they could screen for antibodies to chlamydia. The presence of antibodies in the blood is simply a way of telling whether a person has ever been exposed to a disease.

Chlamydia is a potentially devastating venereal disease that usually has no symptoms, but can leave the people who get it infertile. While doctors knew about chlamydia in the 1980's, they did not know at the time how damaging it could be.

Again, David Hubacher: "And we discovered that women who had antibodies present were twice as likely to have tubal infertility as women who did not. And so the gist of our study is that bacteria, such as those transmitted sexually, cause PID (pelvic inflammatory disease)and subsequent infertility [infertility afterwards]. A plastic device with copper on it does not [cause infertility]."

The Dalkon Shield was the one exception among IUDs that increased the risk of PID, or pelvic inflammatory disease, because it helped introduce bacteria into the uterus. Its braided retrieval cord actually made it easier for bacteria, such as chlamydia, to hitch a ride from the vaginal canal into the womb. Lawsuits against the maker of the Dalkon Shield were numerous. And U.S. federal regulators eventually banned it from the market.

Dr. Deena Kleinerman: "The bottom line of the study is do not blame IUDs for infertility. Blame chlamydia for infertility."

Deena Kleinerman is an obstetrician and gynecologist in private practice in Washington. Dr. Kleinerman says no one method of contraception is right for everyone, and that goes for the IUD. "For the woman who is in a long term, stable monogamous relationship, who is not a candidate to pick up sexually transmitted diseases, and who is an otherwise healthy person, I think that is a person who is very appropriate to consider the use of an IUD," Dr. Kleinerman said.

The study on copper IUDs and chlamydia is published in the New England Journal of Medicine.