NEW YORK - The U.S. abortion rate dropped significantly during the second half of the 1990s, particularly among teenagers, and experts attribute the decline to better awareness of contraception and a fear of disease that has cut down on sexual activity.

The rate fell 11 percent between 1994 and 2000, from about 24 abortions for every 1,000 women of childbearing age to 21, the nonprofit Alan Guttmacher Institute reported Tuesday. The rate among girls ages 15 to 18 declined a dramatic 39 percent, from 24 abortions per 1,000 girls to 15.

At the same time, researchers were surprised by a sharp increase in abortions among low-income women, or those who earn less than twice the federal poverty level of about $17,000 for a family of four.

"Their abortion rates were increasing while they were going down for everyone else," said Rachel K. Jones, who led the study, which was based on questionnaires completed by more than 10,000 women who had abortions.

Sarah Brown, director of the National Campaign to Prevent Teen Pregnancy, noted that the drop in abortions was accompanied by a decline in teen sex, teen pregnancies and teen births during the late 1990s.

"This signals a deep and profound and robust change in adolescent sexual behavior in this country," she said. "I think it's cause for -- I don't know if 'celebration' is the right word -- but certainly our full attention."

Analysts have credited a broad set of factors for those trends, including fear of AIDS and other sexually transmitted diseases and more open discussion with youngsters about sex.

"People are really aware, and we talk more about abstinence and staying away from it altogether," said Shannon Kilcoyne, 18, a high school senior from Greenville, S.C. Kilcoyne was not aware of the study, but said the findings about teenagers reflect concerns about sexual activity among her peers.

"It's more a fear of STDs," she said.

Researchers said more money for teen pregnancy prevention programs has probably improved awareness and access to contraceptives. Similarly, they said that less money for family planning programs for poor women could be one factor for the increase in their abortion rate.

For women below the poverty line, the abortion rate rose 25 percent. It climbed 23 percent among women making less than twice that level.

"There have been more and more restrictions on funding for abortions and in some instances, family planning and contraceptive services," said Kathryn Kolbert, a legal expert on reproductive rights at the University of Pennsylvania's Annenberg Public Policy Center.

Laura Echevarria, a spokeswoman for the National Right to Life Committee, questioned whether the increase in the abortion rate among poor women had anything to do with a lack of access to contraceptives.

"I'd like to see what their educational levels are, how many of them have access to educational material, how many of them understand childbirth," she said.