In the summer of 2014, after the Islamic State group, also known as ISIS, overran the northern Iraqi town of Sinjar, thousands of Yazidis — who practice an ancient religion predating Islam and Christianity, and are considered “devil worshippers” by the Sunni militant group — were killed, raped and enslaved. While most of the surviving Yazidi population now lives in makeshift camps in the Kurdistan region, an unknown number of Yazidi women are still being held captive by ISIS.

According to a report by the New York Times, which cited interviews with over three dozen Yazidi women who escaped captivity, ISIS militants used several forms of contraceptives to ensure that these women did not get pregnant. This was done to comply with the group’s interpretation of a provision in the Islamic law Sharia, which prohibits the sale of, and sexual intercourse with, a pregnant woman.

Nezar Ismet Taib, a doctor who heads the ministry of health directorate in Dohuk, Iraq, which oversees the United Nations-backed clinic where the victims are being treated, told the Times that of the 700 women raped by ISIS militants, only 35, or 5 percent, were found to be pregnant.

He said that since the normal fertility rate for a young woman is between 20 and 25 percent in any given month, at least 140 cases should have been reported. A medical study of 68 women raped in Croatia and Bosnia and Herzegovina during the Yugoslav War, for instance, found that 29, or over 42 percent, had become pregnant.

“These women [the Yazidis] had multiple partners and were raped every day over many months,” Taib told the Times. “I concluded that either they did an abortion before they came back or they used contraception. And if there were abortions, then there would have been physical signs. ... There were no signs.”

The report said that ISIS used both, oral and injectable contraception, to prevent pregnancies, and, in at least one case, a woman was forced to have an abortion in order to “make her available for sex.”

Abduction of women and girls and systematized sale of sex slaves in “markets” has become the hallmark of ISIS’ rule in the territories it still controls. According to a December 2014 report by the human rights group Amnesty International, rape and torture at the hands of militants has driven many captives to suicide.

“The IS [ISIS] has not tried to hide or deny its crimes. On the contrary, more than any other group, it has gone to great efforts to publicize gruesome details of the atrocities perpetrated by its members against captured soldiers and civilians alike, giving them ample resonance through videos and statements widely distributed on social media, often in multiple languages,” Amnesty said in its report.