HARRISBURG, Pa. -- Teenagers and young women who use birth control pills can't blame their tight jeans on the contraceptive, say researchers who have done a long-term study.

The study of 66 women who were followed through their teen years to age 21 showed that the pill-users and nonusers were not significantly different from each other in body weight or percentage of body fat.

"A nonevent here is important," said Tom Lloyd, one of the authors of the study. "The No. 1 reason for women abandoning oral contraceptive use is their perception that they are getting fatter. However, in our study and in previous studies with adult women, that is not true."

Lloyd said the study, conducted by Penn State Milton S. Hershey Medical Center, provides physicians with a significant piece of information that they can use in counseling patients to stay on the contraceptives.

The study, reported recently in the journal Obstetrics and Gynecology, showed pill users did have higher cholesterol than nonusers. But their numbers still fell within the normal range.

Part of longer study

The research on young women and the pill is part of a larger, ongoing study begun in 1990 with the enrollment of 112 healthy adolescent girls from central Pennsylvania.

Researchers took blood samples and X-ray measurements from the participants and tracked their body composition through their teen years. Participants answered questions about physical activity so researchers could adjust results for their level of activity.

Of the 66 women, 39 used birth control pills for an average of 28 months and a minimum of six months; all still used the pill at age 21. The 27 nonusers had never taken oral contraceptives.

One researcher said the study confirms what doctors have known for years: There is no evidence that the birth-control pill causes weight gain in women.

"The benefit here is the duration of the study," said Dr. David F. Archer, professor of obstetrics and gynecology at the Eastern Virginia Medical School in Norfolk, Va., and director of the school's clinical research center. The study's weakness is the small number of participants, he added.

Only white women

Archer, who was not involved in the study, also noted the research included only white women, and "it would always be better to have a study more representative of the population."

What is seen as weight gain due to the pill may really just be the result of natural causes, said Dr. Daniel Mischell of the University of Southern California School of Medicine.

"In older women and also in young girls, they're going to gain weight as they get older," Mischell said. "They like to suggest it's the pill they're taking that's causing it."

The study's findings apply equally to body shape as they do to weight, said Dr. Richard Legro, co-author of the study.

"Your bikini's going to fit the same way every year you're on the pill," said Legro, an associate professor of obstetrics and gynecology at Penn State University College of Medicine.