'He became a different person,' she said. 'He beat me even though I was pregnant. He said he didn't want a wife who bore only girls so he told me to get an abortion or leave his house.'

On the eve of World Population Day on Monday, Vietnam is being forced to confront a growing problem - a preference for sons is destroying the country's gender balance and threatening to cause instability.

It's a problem which has been acknowledged for many years in Asian countries, especially in China and India, but only appeared a few years ago in Vietnam.

At a typical Hanoi kindergarten, teacher Nguyen Thi Quyen has noticed a dramatic change over the past five years.

'This year, my class has 30 children,' she said. 'Twenty are boys and only 12 are girls. Many of them will not be able to find a wife when they grow up.'

A decade ago, the gender birth ratio equalled the accepted international average of around 104 boys to 100 girls. However, thanks to easier access to clinics which determine foetal sex and conduct abortions, the number of male births is now well ahead of female births.

According to the Ministry of Health, there are currently 111 boys born for every 100 girls. But in many regions the number of boys born exceeds those of girls by 20 per cent or more.

'The ratio of gender disparity in Vietnam has reached an alarming level,' Duong Quoc Trong, director of the General Office for Population and Family Planning, told the German Press Agency dpa.

It is illegal for medical staff to tell expectant parents the gender of their unborn child. To get around this, doctors tell pregnant women they will have a 'bird' or a 'butterfly.'

The practice could have very damaging long-term effects, Ian Howie, the United Nations Population Fund's Vietnam representative, told dpa.

'I believe the imbalance in the sex ratio at birth can have a number of impacts on social and cultural norms,' he said.

It may for example increase demands for sex workers and therefore an upsurge in human trafficking, as has been seen in China, South Korea and Taiwan.

It can also cause problems for economic development and increased social instability as a growing population of men searches for partners.

Trong said the rate was set to increase to 115 boys for every 100 girls by 2015, leading to 3 million more men than women by the time the generation reaches marrying age.

The preference for boys in Vietnam means the country now has the third highest abortion rate in the world.

Abortion services are provided in both public and private hospitals with each abortion costing about 20 dollars. On average, every Vietnamese woman has 2.5 abortions in her lifetime.

Predictably, the number of female foetuses aborted greatly exceeds the number of male foetuses.

There are no official figures on how many Vietnamese people seek health treatment overseas, but the government on July 1 said that the Vietnamese spent about 1 billion dollars on health treatment in Singapore last year.

Research by dpa also shows that that thousands of Vietnamese people have sought treatment abroad in recent years, including trips to ensure they give birth to baby boys.

In Hanoi alone, there are at least five travel agencies that act as agents for foreign hospitals to help couples have baby boys. Many of them belong to Thai hospitals.

One agent told dpa that each couple spends on average 20,000 dollars on in vitro fertilization (IVF) treatment to ensure a male baby. Around 20 couples sought help from her agency each year, she said.

Vietnam's patriarchal culture explains why many couples are willing to go to these lengths, Khuat Thu Hong, director of Hanoi's Institute for Social Development Studies, said.

'Men want a son to continue the family line and support them in old age,' she said.

The issue has led to a rise in divorce rates and family violence as men seek other women apart from their wives to bear sons, she said.

Vietnam's latest divorce figures show almost 88,000 divorces were recorded last year in comparison to 84,300 in 2009 and 60,500 in 2005.

'It will take perhaps 50 years to persuade them to stop choosing boys instead of girls,' said Khuat. 'Lan's case is not the only one in Vietnam.'

That is little comfort to Lan, who looks pensive as she rocks her baby daughter in her arms.

'I have heard my ex-husband re-married and his wife is pregnant,' she said. 'I don't know if she will end up in the same situation as me. May God protect her.'

By Pham Bac - Deutsche Presse Agentur - July 10, 2011