Girl under 15 married every seven seconds, says Save the Children - BBC News
The study also studies girls' situations around the world, explores ways to empower and give voices to girls, examines the importance of sexual and reproductive health services and more.
Read more below and at the link, or read the Save the Children study.
The study says girls as young as 10 are forced to marry much older men in countries including Afghanistan, Yemen, India and Somalia.
Save the Children says early marriage can trigger a cycle of disadvantage across every part of a girl's life.
Conflict, poverty and humanitarian crises are seen as major factors that leave girls exposed to child marriage.
"Child marriage starts a cycle of disadvantage that denies girls the most basic rights to learn, develop and be children," said Save the Children International CEO Helle Thorning-Schmidt.
"Girls who marry too early often can't attend school, and are more likely to face domestic violence, abuse and rape. They fall pregnant and are exposed to STIs (sexually transmitted infections) including HIV."
The report, called Every Last Girl, ranks countries based on the hardest place to be a girl based on schooling, child marriage, teen pregnancy, maternal deaths and the number of women in parliament.
Chad, Niger, Central African Republic, Mali and Somalia were ranked at the bottom of the index.