The 19-year-old, identified as Hamshe, said she and her baby were held for 28 days by IS militants, who she believes killed her husband, brother-in-law and father-in-law.
She spoke to activist and former journalist Nareen Shammo as part of a BBC Arabic documentary, Slaves of the Caliphate, in which Shammo tracks captured Yazidi women and works to negotiate their release and return home to northern Iraq.
The Yazidi are a religious sect living mainly in northern Iraq whose beliefs combine elements of several ancient Middle Eastern religions. Thousands have been killed or captured since an IS offensive in the region began last August.
Since then hundreds of Yazidi women and girls have been captured, raped and tortured, and forced to convert to Islam and marry IS followers, according to rights groups Human Rights Watch and Amnesty International.
Hamshe recalled the militants separating their male and female prisoners and said it "was very painful to witness women and girls being taken as spoils of war."
A pamphlet published in December by IS, a Sunni Muslim Islamist group that has declared a caliphate in parts of Iraq and Syria it controls, said it was permissible to buy, sell or give women and girls away as gifts, the documentary said.
"They forced the Yazidi girls to donate blood to IS wounded fighters. Which God allows these acts?" it added.
Hamshe said she escaped at night while her captors were sleeping outside the room she was being held in. She took her baby and walked for four hours before being found by an Arab man who helped her return to her family.
Another victim said she saw girls being raped and tortured, babies separated from their mothers, and young children taken from their families.
She said one of the leaders took a 13-year-old girl to his house and raped her repeatedly over three days, having told his children that she was a convert to Islam, who he would teach to pray and read the Koran.
"There is no horror I haven't experienced. I lost my senses. There is nothing worse than rape," the 21-year-old told Shammo.
An Amnesty report published in December said that IS was seizing girls as young as 12, and that many women and girls had attempted or contemplated suicide to escape the horrors of captivity and sexual violence.
BBC Arabic estimates that 300 Yazidi women out of more than 3,000 captured have been released since August last year.
Shammo, who contacts captured women via mobile phones still in their possession, told the BBC Arabic investigation it was the first time she had heard of girls' and women's blood being used for transfusions.