Low dose of mifepristone used postcoitally is effective in preventing pregnancy
Estrin, D J
As little as 10 mg of mifepristone is effective as an emergency contraceptive treatment, even if the delay between unprotected intercourse and treatment is as long as five days. According to a recent study, the pregnancy rate among women who had no further acts of unprotected intercourse after the one that prompted treatment was 0.9%, regardless of dosage.l In addition, the proportion of women whose menses were delayed for more than seven days after taking mifepristone among those who received 10 mg was half that among women who received 600 mg (18% vs. 36%).
The study was carried out in 11 family planning clinics in Australia, China, Finland, Georgia, the United Kingdom and the United States. Women who requested emergency contraception within 120 hours of a single act of unprotected coitus in the current menstrual cycle were eligible to participate if they were healthy, had regular menstrual cycles of 24-42 days, were willing to abstain from further acts of intercourse and would be available for follow-up over the next six weeks. Those who had recently discontinued hormonal contraception or had been pregnant were included if they had had at least one normal menstrual cycle before the current cycle.
Women were excluded if they were currently pregnant or breastfeeding, had used hormonal contraception in the current menstrual cycle, were uncertain about the date of their last menstrual period or had contraindications to the use of mifepristone. Finally, women were excluded if they said they would continue their pregnancy in the event that emergency contraception failed. Study participants provided gynecologic, obstetric and medical histories and were advised to abstain from intercourse or to use a barrier contraceptive until their next menses.
The 1,717 women enrolled were allocated randomly to receive 10 mg (572 women), 50 mg (575), or 600 mg of mifepristone (570). Of these women, 32 were lost to follow-up and one was found to have been pregnant when treated; the final sample for analysis therefore included 1,684 women.
A total of 20 pregnancies occurred among the women included in the analysis-seven among women who received 10 mg of mifepristone, six among those who received 50 mg and seven among those who received 600 mg. By comparing these observed numbers of pregnancies to the number that would have been expected* in each group in the absence of treatment (48, 43 and 45, respectively), the investigators calculated that mifepristone prevented 85%, 86% and 84% of pregnancies, respectively
The pregnancy rate for the 1,651 women who had no acts of intercourse after the one that prompted them to seek treatment or who used a barrier method during sex was 0.9% for each dosage group. Among the subset of 943 women who had no further acts of intercourse, only five women became pregnant; the pregnancy rate was 0.3% for those who received 10 mg or 600 mg and 1.0% for those who received 50 mg. Compared with women who had no further acts of coitus, those who had intercourse after treatment were almost four times as likely to become pregnant (relative risk of 3.6). For women who had no further acts of intercourse, the risk of pregnancy among those who received treatment more than three days after coitus did not differ significantly from the risk among those who obtained earlier treatment, although the number of pregnancies observed was too small to allow definitive conclusions to be drawn.
The dose of mifepristone was significantly related to the length of the delay in the onset of the next menses and to the incidence of bleeding within five days of treatment (a common side effect of mifepristone during the latter part of the cycle). The proportion of women whose next menstrual period was delayed more than seven days was 36% after 600 mg, 23% after 50 mg and 18% after 10 mg. The incidence of bleeding within five days after mifepristone treatment rose with dosage, from 15% among women who received 10 mg to 31% among those who received 50 mg and 35% among those who received 600 mg. The proportion of women who experienced side effects (fatigue, weakness, nausea, headache, dizziness or vomiting) after taking mifepristone was not significantly related to dosage.
According to the investigators, their data show that using lower doses of mifepristone would provide emergency contraceptive treatment that was less disruptive to the menstrual cycle as well as being substantially less expensive. They conclude, therefore, that “lowering the mifepristone dose from 600 mg to 10 mg has important advantages without significantly compromising effectiveness.”-D.J. Estrin
*Numbers of expected pregnancies were calculated by multiplying the number of women having unprotected intercourse on each day of the menstrual cycle by the probability of conception on that cycle day.
1. Task Force on Postovulatory Methods of Fertility Regulation, Comparison of three single doses of mifepristone as emergency contraception: a randomised trial, Lancet, 1999, 353(9154):697-702.
Copyright Alan Guttmacher Institute Sep 1999
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