Almost 7 percent of girls from poor families in northwest China drop out of education early, a study has found.

Researchers surveyed 11,000 students at 46 middle schools between 2010 and early this year, with the results published in time for the first International Day of the Girl Child, which fell on Thursday.

According to experts, a major contributing factor in the drop out rate is teenage marriage.

"Girls often enter early marriages, at the expense of learning, due to pressure from their families," said Xie Lihua, director of the Cultural Development Center for Rural Women.

In China, the proportion of early marriages fell by 35 percent in the 1970s, but rose from 13 percent in 1979 to 18 percent in 1987, China Population Today, an English-language magazine, reported in 1991.

More than 30 percent of girls in developing countries are married before 18, with about 14 percent married before 15, studies show.

Early marriage raises the risk of early pregnancy and poor reproductive health, as well as perpetuating the cycle of under-education and poverty, according to the World Health Organization.

Globally, about 75 million girls are estimated to miss out on education, said Plan International, a children's organization, in its Learning for Life report.

The group said many still drop out before reaching secondary school level.

"Even when most families in a town or village no longer suffer poverty, inadequate education still confronts girl children," said Luo Zhaohong, president of Practical Skills Training Center for Rural Women.

"Because of the lack of a good role model, they voluntarily quit schooling to earn money, even though the nine-year compulsory education system supplies enough educational resources."

Feng Yuan, from the Anti-domestic Violence Network in Beijing added: "Domestic violence results in girl children dropping-out and leaving home at a very young age."

She said, based on her experience, many young, female migrant workers are victims of domestic violence.