More than 700 million women alive today were married as children, and about 250 million of them — more than one in three — were wed before they reached 15, Unicef said in a study released Monday, on the eve of an international meeting in London, the Girl Summit, sponsored by Unicef and the British government. It aims to mobilize efforts to end female genital mutilation and child, early and forced marriages. The study said child marriage remains widespread in parts of South Asia and sub-Saharan Africa. South Asia is home to almost half of all child brides worldwide, the study found. While an adolescent girl today is about a third less likely to be subjected to genital mutilation compared with 30 years ago because of activism, legislation and changing attitudes, Unicef said that in countries where it is still practiced, parents “continue to compel their daughters to undergo the procedure because of strong social pressure.”