By Melissa Schorr

NEW YORK, Mar 29 (Reuters Health) - A new study by Canadian researchers questions the conventional wisdom that women who have had unprotected sex must use emergency contraception within 72 hours to prevent an unwanted pregnancy.

The investigators found that taking so-called "morning-after" pills 3 to 5 days after having unprotected sex was up to 87% effective in preventing pregnancy, while receiving the treatment within 3 days, as has been recommended for decades, was up to 90% effective.

"Women should be encouraged to seek consultation as soon as possible," according to Dr. Isabel Rodrigues, a professor of family medicine at the University of Montreal in Quebec, Canada, and colleagues. "However, after 72 hours, the regimen...has a favorable effectiveness rate and therefore should be recommended."

The study is published in the March issue of the American Journal of Obstetrics and Gynecology, and was funded by the Federation des Medecins Omnipracticiens du Quebec.

The researchers hoped to reveal whether emergency contraceptives have a longer window of effectiveness than 72 hours after sex. Once a woman has passed that initial time frame after unprotected sex, her options for terminating a pregnancy narrow. These options include placement of an IUD for up to 5 days after unprotected intercourse, surgical abortion or the abortion pill RU-486, which is still not legal in some countries, including Canada.

"We have long stated that the 3-day time window is too restrictive," Princeton professor James Trussell, an expert on emergency contraceptives, told Reuters Health. "Women should be told that after that time, taking emergency contraceptives might do some good and would do no harm.

"The greatest harm is clinicians not giving it out after 72 hours," he added. "Maybe this will help."

Rodrigues and colleagues followed 300 women who visited 26 community clinics in Quebec to receive family planning services. The average age of the women was 18 and the vast majority had never given birth.

The researchers tracked two groups: 131 women who arrived at the clinic within 3 days of having unprotected sex and another 169 women who arrived within 3 to 5 days after having unprotected sex.

All were given emergency contraception, a combination of estrogen and progestin taken on the spot and 12 hours later. The investigators followed up 3 weeks later to determine if any of the women had become pregnant.

The pregnancy rate for the women taking the emergency contraceptive within 72 hours was 0.8%, compared with 1.8% for the women taking the pills within 3 to 5 days.

Both groups of women had significantly fewer pregnancies than would be expected if no emergency contraception was used, although the small number of women in the study who became pregnant made it hard to draw firm conclusions, the authors note.

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