A new decree details seven scenarios in which couples are to have the right to a third child, Nguyen Thi Thanh Huong, director of the Hanoi Population Department, said.

Parents who already have one child and are taken over the two-child limit by twins or triplets in the second pregnancy, will not be considered to have violated the policy.

Couples with children from prior marriages or with disabled or deceased children, and members of ethnic minorities numbering fewer than 10,000 people are also to be allowed to have a third child.

Finally, mothers with two children out of wedlock are authorized to have a third if they get married.

"This is the first time the government has given out detailed guidance on who can have a third baby," Huong said.

As a practical matter, Vietnam's population policies have long been flexible. Since signing on to international agreements in 2003 that protect the reproductive rights of parents, the government has "encouraged" two-child families rather than mandating them legally.

Even when the policy was more stringent, violations were generally punished only with subtle administrative measures. Parents who worked in state-owned enterprises might find themselves denied promotion, for example.

According to the United Nations Population Fund (UNFPA), Vietnam's birth rate has been declining naturally due to rising use of contraceptives and the tendency to have fewer children as income levels rise. Total fertility stood at 2.09 children per woman in 2008, slightly below the replacement rate.

The 2008 figures showed 16.9 per cent of Vietnamese women had more than two children.

"In the public domain there's very little reference to the two-child policy anymore," said Bruce Campbell, UNFPA's representative in Hanoi. "There's more reference to the 'small-family norm'."

Campbell said that Vietnam's birthrate would likely follow the pattern of other East Asian countries, like Japan and South Korea, which have seen fertility plunge far below the replacement rate as prosperity grew.

He anticipated that Vietnamese policies might even switch from discouraging to promoting higher birthrates in future, to avoid the struggle of supporting the retiree population with a shrinking labour force, as has been seen elsewhere.

The narrow criteria of new regulations apply to too few couples to lead to a significant difference in the birthrate, Campbell said.

The two-child policy was initially introduced to counter a baby boom after the end of the Vietnam War in 1975. At the time, the country faced food shortages, and the effort to hold down the population was considered a government policy success in the 1980s.

In recent years the more pressing population problem has become a disparity in gender ratios. According to Vietnam's Ministry of Health, there are currently 112 boys born for every 100 girls.

Some critics have said that the two-child policy, combined with the strong preference for sons in Vietnam's Confucian culture, leads parents to abort female foetuses.

Other East Asian countries with no official family-size policies, including South Korea, have faced even sharper gender imbalances at similar stages of development.

Vietnam's population of 86 million makes it the 13th-most-populous country in the world.

Deutsche Presse Agentur - March 10, 2010