More than 14 million girls under the age of 18 will be married each year during the next decade, and the number could increase to more than 15 million between 2021 and 2030, a new report from the UN Population Fund predicts.
As the number of young married females grows, the number of children bearing children will increase, along with deaths among girls, it added.
Child marriage is a violation of fundamental human rights, according to international conventions. Early marriage often leads to early departure from school, economic dependency and difficulty finding work outside the home.
It can also perpetuate poverty and increase susceptibility to sexually transmitted diseases such as as HIV/AIDS.
"In those communities where the practice of child marriage remains common, families can feel it's not worth investing at all in the education of their daughters," said Babatunde Osotime-hin, the fund's executive director.
Osotimehin, who last month launched a global campaign urging countries to invest in the education of women and young people, said uneducated girls are more than three times more likely to marry before age 18 than those with secondary education or higher.
"No country can afford the lost opportunity, waste of talent or personal exploitation that child marriage causes," she added in an interview.
In 2010, 158 countries reported that 18 was the minimum legal age for girls to marry without parental consent. But in 146 countries, girls younger than 18 marry with the consent of parents, and in 52 nations the age is under 15.
In 41 countries, 30 per cent or more of women 20-24 years old had married by age 18, with Niger topping the list at 75 per cent.
