The number of abortions fell by 110,000 between 1994 and 2000, to 1.3 million, and use of emergency contraception was credited with 43 percent of that decline, according to the survey that appears in the latest issue of Perspectives on Sexual and Reproductive Health.
The hormone pills, which prevent a fertilized egg from implanting in the uterus for up to 72 hours after intercourse, have been available for decades. But family planning groups have been promoting their use heavily since the mid-1990s as a way to avoid unwanted pregnancies.
Dr. Vanessa Cullins, vice president for medical affairs at the Planned Parenthood Federation of America, said the new study underscores the importance of the morning-after pill as a safety net.
"We want every woman at risk to have access to emergency contraception when a contraceptive mishap occurs," Cullins said.
Emergency contraception is about 75 percent effective at preventing pregnancy, she added.
The pills -- either a combination of estrogen and progestin or progestin alone -- don't trigger an abortion. In fact, emergency contraception won't work once a pregnancy has occurred.
Women occasionally take the hormones after becoming pregnant, but they face no higher risk of miscarriage or birth defects than those who haven't used the drugs, Cullins said.
The survey, by scientists at the Alan Guttmacher Institute, a reproductive health group in New York City, included 10,683 women who'd had an abortion in 2000. Of those, 1.3 percent said they'd used emergency contraception that failed, prompting them to seek an abortion.
Applying that rate to the total number of abortions in 2000 (1.3 million) suggests that almost 17,000 women had abortions after first trying emergency contraception. If the pill succeeds three times for every failure, as the experts say, then at least 51,000 abortions were averted.
Slightly more than half of the women in the survey were in their mid-20s, 19 percent were adolescents, and 17 percent were married. All were asked about their history of contraceptive use in the month before the abortion.
Fifty-four percent said they'd been using some type of contraception around the time they got pregnant. (That was up from 51 percent in 1987, though somewhat lower than the 58 percent in 1994.) Of those, 28 percent had relied on their partner to wear a condom -- vs. 32 percent in 1994 -- while 14 percent were taking oral birth control pills.
Young women and teens were less likely than older women to use any form of birth control, the survey found. Poor women used contraception less often than their wealthier counterparts, and minorities were less likely to do so than whites.
Rachel Jones, a sociologist at the Guttmacher Institute and a co-author of the report, said the findings don't suggest contraception isn't effective. Rather, she said, improper use of the methods weakens their protection.
Seventy five percent of the women on the pill and 49 percent of condom users said they'd been using the methods inconsistently when they got pregnant. More than four in 10 condom users also said the device had slipped or torn during intercourse.
"In this population of women having abortions, problems with contraceptives are much more pronounced" than in the rest of the sexually active public, Jones said.
When used correctly, birth control pills prevent better than 99 percent of pregnancies, while condoms can block 97 percent of pregnancies over a year in couples who use them as directed.
The survey also revealed that many girls and women have unrealistic attitudes about sex and birth control.
One in three who weren't using contraceptives said they didn't think they'd get pregnant, while another third said concerns about how to use birth control kept them from doing so.
Another 27 percent of the women and 44 percent of the teens said they weren't expecting to have sex when they did.
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