Thousands of vulnerable women and girls from northern Myanmar are being trafficked to China and forced to marry, a study said on Friday, offering a rare look at an issue that grips the conflict-hit borderlands.

China has around 33 million fewer women than men due to the decades-long one-child policy.

To plug the gap, tens of thousands of poor women from Cambodia, Laos, Myanmar and Vietnam are sold as brides each year, some going willingly, while others are tricked or trafficked.

In the first research of its kind, a report by the Johns Hopkins Bloomberg School of Public Health estimated 7,500 women from war-torn Kachin and northern Shan states have fallen victim to forced marriage in China.

Based on interviews with scores of people who escaped and returned to Myanmar, and others still inside China, the study found that the majority of those trafficked were also forced to carry a child for their husband.

Women leave Myanmar because of “conflict, displacement and poverty” said report author W. Courtland Robinson, while “the male-female sex imbalance in China, especially in rural areas” means demand for a wife is high.

One woman told researchers that she was trafficked into China three times, and each time “pushed into giving birth”, said Moon Nay Li of Kachin Women's Association Thailand, who led the field research in Kachin and Shan states.

“Because of political instability, conflict and land confiscation... security for women is a big challenge,” she said.

Marriages are often arranged and brokered by the women's own families and village elders, with brides-to-be unable to refuse as they are at the bottom of the social hierarchy.

The youngest women command higher prices of up to $10,000-15,000.

Their matches in China are typically to older, sick, or disabled men in rural areas — people considered undesirable to the Han Chinese — while the women's lack of documentation plunges them into a legal limbo.

The study found that women were often “hired” to bear children for their husbands and then forced out or sold on to new husbands to recoup money.

Some women have successful marriages and the issue of consent is complex and varies case-by-case, Robinson said.

But all unions should be entered into without “threat, menace or penalty”, he added.

Often daughters “can't say no to their parents”, said Moon Nay Li, and feel compelled to go ahead with the marriage once “traffickers and agents have given money to their parents”.

Researchers called for Myanmar to end the conflict in Kachin and Shan states, which has displaced tens of thousands of women, and to train anti-trafficking officials to enforce the law and recognise the women as victims.