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 over all, 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 gynecologist carrying out the examinations.

It is a stunningly low figure given that the normal fertility rate for a young woman is between 20 percent and 25 percent in any given month, four to five times the rate that has been recorded so far, said Dr. Nezar Ismet Taib, who heads the Ministry of Health Directorate in Dohuk, which oversees the clinic where the victims are being treated.

“We were expecting something much higher,” he said.

The captured teenage girl, who agreed to be identified by her first initial, M., has the demeanor of a child and wears her hair in a bouncy ponytail. She 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, unnerved by what he perceived as a delay, gave her a version of the so-called morning-after pill, causing her to start bleeding.