Furthermore, of the 140 million girls who will marry before the age of 18, 50 million will be under the age of 15.
Despite the physical damage and the persistent discrimination to young girls, little progress has been made toward ending the practice of child marriage.
In fact, the problem threatens to increase with the expanding youth population in the developing world.
"Child marriage is an appalling violation of human rights and robs girls of their education, health and long-term prospects," Babatunde Osotimehin, M.D, Executive Director, UNFPA, said.
"A girl who is married as a child is one whose potential will not be fulfilled. Since many parents and communities also want the very best for their daughters, we must work together and end child marriage," Osotimehin added.
Girls married young are more vulnerable to intimate partner violence and sexual abuse than those who marry later.
"Complications of pregnancy and childbirth are the leading cause of death in young women aged 15-19. Young girls who marry later and delay pregnancy beyond their adolescence have more chances to stay healthier, to better their education and build a better life for themselves and their families," Flavia Bustreo, M.D., Assistant Director-General for Family, Women's and Children's Health at the World Health Organization, said.
If child marriage is not properly addressed, UN Millennium Development Goals 4 and 5 - calling for a two-thirds reduction in the under-five mortality rate and a three-fourths reduction in the maternal deaths by 2015 - will not be met.
Child marriage - defined as marriage before the age of 18 - applies to both boys and girls, but the practice is far more common among young girls.
Child marriage is a global issue, but rates vary dramatically, both within and between countries. In both proportions and numbers, most child marriages take place in rural sub-Saharan Africa and South Asia.
In South Asia, nearly half of young women and in sub-Saharan Africa more than one third of young women are married by their 18th birthday.
The 10 countries with the highest rates of child marriage are: Niger, 75 percent; Chad and Central African Republic, 68 percent; Bangladesh, 66 percent; Guinea, 63 percent; Mozambique, 56 percent; Mali, 55 percent; Burkina Faso and South Sudan, 52 percent; and Malawi, 50 percent.
In terms of absolute numbers, because of the size of its population, India has the most child marriages and a prevalence of 47 percent.