Along with filmed beheadings and terror attacks, sex slavery is another crime ISIS practices and justifies with its extremist ideology. According a new investigation by The New York Times, it is a practice the terror group maintains through a surprising mechanism: birth control.

The Times interviewed many former ISIS sex slaves from the Yazidi minority currently being treated and resettled. The teenage girls, at least one of whom was sold about seven times to ISIS fighters, revealed that their abusers forced them to take either oral or injectable contraceptives, and were also particularly wary of their menstrual cycles.

“Every day, I had to swallow one in front of him. He gave me one box per month. When I ran out, he replaced it. When I was sold from one man to another, the box of pills came with me,” a former sex slave recalled.

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Before being sold between ISIS fighters, sex slaves were required to take pregnancy tests. Some Yazidi sex slaves who tested positive were forced to have abortions before being sold. On top of this, a gynecologist treating the former sex slaves told the Times that of the more than 700 rape victims from the Yazidi ethnic group receiving treatment, a mere five percent became pregnant during their enslavement, compared with the 20 to 25 percent general fertility rate for women.

The reason for ISIS's emphasis on ensuring its sex slaves don't conceive is rooted in a horrifying law. According to the extremist group's ideology, pregnant sex slaves cannot be raped or sold. Following this logic, it's likely that birth control and abortion are used as means to keep ISIS's trade of sex slave in circulation and continue to enable rape.

This is consistent with an official pamphlet released in 2014 explaining ISIS sex slavery rules. One rule states that a man cannot sell his sex slave if "she becomes the mother of a child." A fatwa (religious ruling) from January 2015 provided the rationale for the extremist group's pervasive sex slave trade, identifying ownership of them as the “graces which Allah has bestowed upon… the Caliphate is the conquest of large surface areas of the country and one of the inevitable consequences of the jihad… is that women and children of infidels will become captives of Muslims.”

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The Washington Post also identified ISIS's use of sex slavery as a source of revenue, selling captured women and girls for income to continue to expand its operations. This helps to explain the urgency with which it allegedly attempts to keep sex slaves from conceiving, so that they can continue to be sold.

The women interviewed by The New York Times were not the first to speak out about their experiences as sex slaves of ISIS fighters. Since ISIS forces raided Sinjar, a city in northern Iraq, and captured over 5,000 Yazidi women in August 2014, those who escaped appeared in a photo series by female Iraqi photographer Seivan Salim called "Escaped."

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Women involved in Salim's project also shared similar, harrowing stories about the abuse they faced, providing disturbing insight into just how ingrained slavery and sexual abuse are in territories conquered by ISIS. As the group's ideology continues to grow and spread in the Middle East, it is likely that more women are suffering from abuse.

h/t The New York Times