Five women have died after treatment with the abortion pill, raising new fears about the risks of the medical method of terminating pregnancy.
Four of the deaths occurred in the United States to young, healthy women who had successful terminations but developed infections within a few days that progressed rapidly, causing septic shock. The fifth death was in Canada.
The US Food and Drug Administration issued a "black box" warning in July alerting doctors to the deaths - details of which are published in the New England Journal of Medicine today - and warning of the risks.
The American women were aged 18, 21, 22 and 34. Three died within hours of being admitted to hospital and the fourth was dead before medical help reached her.
All the fatalities have occurred since 2001.
Investigation of the US deaths showed that all four women were infected with the bacterium Clostridium sordellii, which exists naturally in the genital tract but in rare cases can cause toxic-shock syndrome following childbirth.
Researchers from the Centres for Disease Control in Atlanta say the risks of the abortion pill are low. Since it was approved in the US in 2000, it has been used to induce 460,000 abortions.
Michael Greene, professor of obstetrics at Harvard Medical School, agrees the risk is low. But he says that, at one in 100,000, the risk of death from infection is 10 times higher than the risk from surgical abortion at the same stage of pregnancy (0.1 in 100,000).
"As tragic as the deaths of these healthy young women are, they remain a small number of rare events without a clear pathophysiologic link to the method of termination," said Professor Greene. "Patients should be informed of this risk before they consent to the procedure and should be vigilant for symptoms after the procedure."
He warned regulators against overreacting to "scant data" by stopping prematurely "the only approved medical option for pregnancy termination".