The Islamic State in Iraq and Syria is using birth control to ensure an adequate supply of sex slaves, according to a Saturday report in the New York Times.

Over three dozen Yazidi women who survived Islamic State captivity told the paper the group's fighters use a variety of birth control methods, "including oral and injectable contraception, and sometimes both," to allow for the captives' continued rape. ISIS leadership formally disallows the rape of women who are possibly pregnant to avoid confusion over paternity, though according to the Times, some fighters are "either ignorant of the injunction or defiant of it."

The use of birth control appears to be a way for ISIS fighters to circumvent that technicality, which would otherwise slow the trade in slaves.

Dr. Nezar Ismet Taib, head of the Ministry of Health Directorate in Dohuk, Iraqi told the Times he would normally expect at least 140 pregnant women among some 700 recorded cases of rape, but has only seen 35.

"I concluded that they either did an abortion before they came back or they used contraception," Dr. Taib said. "And if there were abortions, then there would have been physical signs. There were no signs."

ISIS' trafficking in sex slaves is a well-documented phenomenon throughout the territories the group controls in Iraq and Syria. In December, a leaked document revealed a long list of religious rules for the group's systemic abuse of women, including prohibitions on sex with menstruating women and sharing sex slaves with direct relatives.

The group's propaganda has sometimes featured braggadocious passages apparently written for the purpose of luring new members with the promise of women.

"After capture, the Yazidi women and children were then divided according to Sharia [Islamic law] amongst the fighters of Islamic State who participated in the Sinjar operations," an article in the group's magazine Daqib stated, according to the BBC. "Before Satan sows doubt among the weak-minded and weak-hearted, remember that enslaving the kuffa [infidels] and taking their women as concubines is a firmly established aspect of Sharia."

In 2014, the BBC said members of the Yazidi ethnic group, declared heretics by ISIS, estimated the number of women enslaved to be at least 3,500 people.

A military coalition including the U.S. expanded an ongoing air campaign against ISIS in Iraq to include Syrian targets in 2014. Since then, the group has steadily lost ground, and CNN reported German intelligence officials recently obtained a massive cache of data on its members.