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 give a urine sample to be tested for the hCG hormone, whose presence indicates 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 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.