Every three seconds a girl under 18 is married, about 10 million a year.
Mostly it happens without the girl’s agreement and to a much older man in Africa, the Middle East or South Asia, according to figures from children's charity Plan UK.
From shocking injuries caused by childbirth to starving families selling little girls as drought brides to raise cash to buy food, child marriage causes lasting damage, shows a new documentary by the legal news service, TrustLaw.
"Young children have babies - your life is ruined, your education is ruined," said Kanta Devi, who was 16 when she married in Badakakahera village in India's Rajasthan state.
"You become upset with everything in your life," she told TrustLaw.
Making a girl get married before she reaches the age of 18 is a human rights violation according to the UN Convention on the Rights of the Child.
But there are more than 50 million child brides across the world, some as young as eight years old and that number is expected to hit 100 million over the next 10 years, predicts the International Centre for Research on Women (ICRW), a Washington-based think tank.
In Niger, Chad and Mali, more than 70 per cent of girls are married before they hit 18, the ICRW found, looking at data from the past year. And half girls in Bangladesh, Guinea, Central African Republic, Mozambique, Burkina Faso and Nepal marry before 18. In Ethiopia, Malawi, Madagascar, Sierra Leone, Cameroon, Eritrea, Uganda, India, Nicaragua, Zambia and Tanzania, the rate is 40 per cent.
Because they are still growing, girls under 15 are five times more likely to die during pregnancy or childbirth than women over 20, said the US Agency for International Development. Every year 70,000 girls aged 15-19 die during pregnancy or childbirth, UNICEF says. And the UN World Population Fund says pregnancy is the leading cause of death in that age group, because of complications of childbirth and dangerous abortions.
Girls forced to marry before 18 also run a bigger risk of contracting sexually transmitted diseases and HIV/Aids because they are less likely to be able to insist on safe sex.
"Child marriage is a silent health emergency in the sense that it's often overlooked as a root cause of maternal mortality and morbidity (illness)," the ICRW's Jeffery Edmeades said.