Female circumcision, performed on as many as 3 million girls each year, complicates childbirth later in life and causes higher mortality among their babies, the World Health Organization (WHO) said on Friday.

In a new report, the United Nations agency said women who had undergone the practice, also known as female genital mutilation, were up to 70 percent more vulnerable to potentially fatal hemorrhage after delivery than those who had not.

Its study, involving some 28,000 women at obstetric centers in six African countries where the practice is common, said babies born to circumcised women were as much as 55 percent more likely to die during or immediately after childbirth.

“We have, for the first time, evidence that deliveries among women who have been subject to female genital mutilation are significantly more likely to be complicated and dangerous,” said Joy Phumaphi, WHO assistant director-general for family and community health.

Up to 20 out of every 1,000 babies born in Africa die as a direct result of their mothers’ circumcision, the WHO said.

About 100 million women worldwide are estimated to have undergone genital mutilation, which involves cutting off the clitoris and sometimes removing all external genitalia and stitching the vaginal opening to reduce women’s sexual appetite and prevent intercourse.

The WHO said relatively inelastic scar tissue that forms around the wounds could obstruct and prolong labor “which increases the risk of caesarean section, heavy bleeding, distress in the infant and stillbirth”.

The degree of complications increased according to the extent and severity of the procedure.

While predominant in 28 African countries, including Sudan, Chad, Sierra Leone and Djibouti, female circumcision also takes place in some areas of the Middle East and Asia and among immigrant communities in Europe and North America.

Girls generally undergo the rite before the age of 10, often without anesthesia and in unsanitary conditions where they are exposed to dangerous infections including HIV.

Phumaphi said the WHO strongly opposed having doctors perform female genital mutilations to ensure they took place in a clean or safe setting.

Recommending that health professionals be involved in female circumcision would be endorsing an unacceptable practice, she said. “That is the worst possible thing we can do. It is worse than turning a blind eye.”

Many African nations have legislated against female genital mutilation but few are enforcing the rules, Berhane Ras-Work of the Inter-African Committee for Traditional Practices said.

Citing Senegal and Burkina Faso as exceptions, she said most countries in Africa were doing little to stop the widely decried practice meant to ensure girls stay chaste before marriage and remain loyal to their husbands.

In some areas, female circumcision is also seen as a means to initiate girls into womanhood or improve hygiene. While some consider the rite a requirement under the Muslim faith, the WHO denied this, saying the practice predated Islam.