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 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 Osotimehin, UNFPA’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 with TrustLaw, a website covering women’s rights and good governance run by the Thomson Reuters Foundation.
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 get married with the consent of parents, and in 52 nations the age is under 15.
In 41 countries 30 percent or more of women 20-24 years old had married by age 18, with Niger topping the list at 75 percent.
The report said as the 2015 deadline for meeting the U.N. Millennium Development Goals (MDGs) draws near, governments should recognize that tackling child marriage will help meet at least six of the eight anti-poverty targets.
Child marriage hinders gender equality and empowering women. Young brides are often married to men who are much older and, even if they are aware of their rights, cannot claim them.
It also limits progress towards eradicating extreme poverty, reducing child mortality, improving maternal health, combating HIV/AIDS and achieving universal primary education.
“The implications are staggering and demand swift action,” according to the report, which showed that most child marriage occur in South Asia and sub-Saharan Africa.
“Even at lower rates, the absolute number of girls likely to marry before age 18 will remain high as a result of population growth.”
The global population is projected to grow from 7 billion to more than 9 billion by 2050.
The report urged governments to review national legislation and laws to reflect international human rights standards.
Policymakers must also target regions where many young girls are at risk to ensure that family planning, health services, formal education and support for married girls are all available, it added.