DOHUK (Iraq) — Locked inside a room where the only furniture was a bed, the 16-year-old learnt to fear the sunset, because nightfall started the countdown to her next rape.

During the year she was held by the Islamic State, she spent her days dreading the smell of the ISIS fighter’s breath, the disgusting sounds he made and the pain he inflicted on her body. More than anything, she was tormented by the thought she might become pregnant with her rapist’s child.

It was the one thing she need not have worried about.

Soon after buying her, the fighter brought the teenage girl a round box containing four strips of pills, one of them coloured red.

“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,” explained the girl, who learned only months later that she was being given birth control.

It is a modern solution to a medieval injunction: According to an obscure ruling in Islamic law cited by the Islamic State, a man must ensure that the woman he enslaves is free of child before having intercourse with her.

Islamic State leaders have made sexual slavery as they believe it was practised during the Prophet Muhammad’s time integral to the group’s operations, preying on the women and girls the group captured from the Yazidi religious minority almost two years ago.

To keep the sex trade running, the fighters have aggressively pushed birth control on their victims so they can continue the abuse unabated while the women are passed among them.

More than three dozen Yazidi women who recently escaped the Islamic State and who agreed to be interviewed for this article described the numerous methods the fighters used to avoid pregnancy, including oral and injectable contraception, and sometimes both.

In at least one case, a woman was forced to have an abortion in order to make her available for sex, and others were pressured to do so.

Some described how they knew they were about to be sold when they were driven to a hospital to be tested for pregnancy. They awaited their results with apprehension: A positive test would mean they were carrying their abuser’s child; a negative result would allow Islamic State fighters to continue raping them.

The rules have not been universally followed, with many women describing being assaulted by men who were either ignorant of the injunction or defiant of it.

But overall, the methodical use of birth control during at least some of the women’s captivity explains what doctors caring for recent escapees observed: Of the more than 700 rape victims from the Yazidi ethnic group who have sought treatment so far at a United Nations-backed clinic in northern Iraq, just 5 percent became pregnant during their enslavement, according to Dr Nagham Nawzat, the gynaecologist carrying out the examinations.

The captured teenage girl, who agreed to be identified by her first initial, M, was sold a total of seven times.

When prospective buyers came to inquire about her, she overheard them asking for assurances that she was not pregnant, and her owner provided the box of birth control as proof.

That was not enough for the third man who bought her, she said. He quizzed her on the date of her last menstrual cycle and gave her a version of the so-called morning-after pill, causing her to start bleeding.

Finally he came into her room, closed the door and ordered her to lower her pants. The teenager feared she was about to be raped.

Instead he pulled out a syringe and gave her a shot on her upper thigh. It was a 150-milligram dose of Depo-Provera, an injectable contraceptive.

When he had finished, he pushed her back onto the bed and raped her for the first time.

Thousands of women and girls from the Yazidi minority remain captives of the Islamic State, after the jihadis overran their ancestral homeland on Mount Sinjar on Aug 3, 2014. In the months since then, hundreds have managed to escape.

Many of the women interviewed for this article were initially reached through Yazidi community leaders, and gave their consent. All the underage rape victims who agreed to speak were interviewed alongside members of their family.

J. an 18-year-old, said she had been sold to the Islamic State’s governor of Tal Afar, a city in northern Iraq.

“Each month, he made me get a shot. It was his assistant who took me to the hospital,” said J, who was interviewed alongside her mother, after escaping this year.

“On top of that he also gave me birth control pills. He told me, ‘We don’t want you to get pregnant,’” she said.

When she was sold to a more junior fighter in the Syrian city of Tal Barak, it was the man’s mother who escorted her to the hospital.

“She told me, *If you are pregnant, we are going to send you back,’” J said. “About 30 or 40 minutes later, they came back to say I wasn’t pregnant.”

The fighter’s mother triumphantly told her son that the 18-year-old was not pregnant, validating his right to rape her, which he did repeatedly.

A 20-year-old who asked to be identified only as H began to feel nauseated soon after her abduction.

Already pregnant at the time of her capture, she considered herself one of the fortunate ones. For almost two months, H. was held in locked rooms, but she was spared the abuse befalling most of the young women held alongside her.

Despite being repeatedly forced to give a urine sample and always testing positive, she, too, was eventually picked.

Her owner took her to a house, shared by another couple. When the couple was present, he did not approach her, suggesting he knew it was illegal. Only when the couple left did he forcibly have sex with her.

Eventually he drove her to a hospital with the aim of making her have an abortion, and flew into a rage when she refused the surgery, repeatedly punching her in the stomach. Even so, his behaviour suggested he was ashamed: He never told the doctors that he wanted H. to abort, instead imploring her to ask for the procedure herself.

When he drove her home, she waited until he left and then threw herself over the property’s wall.

“My knees were bleeding. I was dizzy. I almost couldn’t walk,” she said.

Weeks later, with the help of smugglers hired by her family, she was spirited out of Islamic State territory.

Her first child, a healthy baby boy, was born two months later. THE NEW YORK TIMES