What kind of place would set the age of consent at 17 – but allow pregnant girls as young as 11 to marry? Florida: its law is less progressive than Afghanistan’s. It is one of 25 US states that allow girls of any age to marry in certain circumstances, such as with judicial approval. Others have minimums set as low as 13.

Though boys too are affected, those affected are overwhelmingly female: almost nine in 10 children who marry are girls, and only rarely do they wed a peer. Almost a third wed men of 21 or older. Marriage is often seen as protecting girls, especially if they are pregnant, but it locks children into abusive relationships. In some states, child brides cannot initiate legal action such as a divorce – or even access refuges – because they are minors. Women who married as children are helping to lead the growing nationwide campaign to ban it, which has already brought change in Connecticut, Texas, Virginia and New York in the past two years. A bill banning marriage under 18 is now going through committees in Florida.

This is a global problem. Worldwide, a girl under 18 is married every two seconds; 250 million women were wed by 15. The results can be devastating. Child marriage is linked to poverty, curtailed education, domestic violence, and maternal mortality. Young brides are under pressure to have children when they still have the bodies of children.

Ending child marriage by 2030 is included in the UN sustainable development goals. The proportion of young women who married before 15 has dropped from 12% to 8% since the early 1980s. But population growth means the absolute numbers will continue to rise. And despite improvements in laws and practice, the situation is deteriorating in places. This month Turkey passed a law allowing Islamic muftis to carry out civil marriage ceremonies; activists there fear conservative scholars could turn a blind eye to child marriages. In Iraq, there has been a renewed attempt to introduce changes allowing children as young as nine to marry. Earlier this year, Bangladesh passed a law allowing girls of any age to marry “under special circumstances” with permission from a court and their parents.

Good laws alone will not protect girls. Many marriages are not formally registered; in other places, officials turn a blind eye to breaches. Even when it was wholly illegal for girls under 18 to wed in Bangladesh, more than half were married by then. Enforcement is key, but so is tackling the underlying factors driving child marriage. These range from patriarchal attitudes through to a lack of economic opportunities, access to contraception, and conflict. Child marriage among Syrian refugees is nearly three times higher than in Syria before the war.

Girls Not Brides, a global partnership of 800 civil society groups, says delaying marriage is in the interests not only of girls but also their communities and their countries: the World Bank says that child marriage will cost developing nations trillions of dollars by 2030.

But laws can provide a basis for protecting girls and send a strong signal. This poses an obvious dilemma for the UK, which – as officials in Bangladesh have pointed out – campaigns globally to end marriage for under-18s, while allowing 16-year-olds in England and Wales to marry with parental consent, and 16-year-olds in Scotland to marry even without it. There is little political appetite for raising the minimum age: the age of consent is 16, the numbers involved are small (around 200 a year in England and Wales) and there is specific legislation on forced marriage. Nonetheless, the case should be carefully considered.

For the US there can be no hesitation. It has seen more than 200,000 child marriages in the past 15 years, with brides as young as 10. Changing state laws to safeguard them would not only protect its own girls but send a powerful global message. Wedlock is a trap for children. Free them.