The General Statistics Office (GSO) of Vietnam said at a conference held on Friday to discuss the 2019 population and housing census that this could reach 2.5 million by 2059.

The study carried out by the GSO together with the United Nations Population Fund (UNFPA) in Vietnam found that over the last 30 years the fertility rate decreased by nearly half from 3.8 children per woman in 1989 to 2.09 last year, below the replacement fertility rate of 2.1.

But the sex ratio at birth was high at 111.5 boys per 100 girls.

The gender imbalance was the highest (115.3) in the Red River Delta and lowest in the Mekong River Delta (106.9).

The problem of gender imbalance was first highlighted in 2004, and has rapidly worsened since.

The "natural" or "normal" sex ratio is 105-106.

The main cause of the gender imbalance is people’s preference for boys over girls, Vu Thi Thu Thuy, director of the GSO's department of population and labor statistics, said.

Couples with two children but no son are twice as likely to have a third child as those with at least one son, she said.

This is particularly evident in families with high educational background and income, she added.

Experts said Vietnam is still heavily influenced by Confucian values, including patriarchy, which favors males over females in family matters and social settings as men are responsible for carrying on the family lineage and worship their ancestors, among others.

Naomi Kitahara, UNFPA representative in Vietnam, expressed concern that Vietnam’s rate of imbalance was the third highest in the world after China and India.

"There is a strong evidence to suggest that this is the result of gender-biased, pre-natal sex selection," she said.

The government needs to create a legal framework to prevent the sex selection of fetus, she said.

The government is working to raise awareness of the importance of a balanced sex ratio and pushing social organizations and communities to act.

Through this campaign, which will go on until 2025, officials hope to bring the ratio below 109 by 2030.

Vietnam's population was 96.2 million at the time of the census, the third highest in Southeast Asia behind Indonesia and the Philippines, and 15th highest globally.

Kitahara was also concerned that "Vietnam's population is aging at an unprecedented rate." The country had 11.4 million people aged 60 or more as of last year, or nearly 12 percent of the population.

She said now is the time to have legal and policy frameworks which are fully in line with the International Conference on Population and Development principles, allowing individuals and couples to freely choose the number of children, time of birth and time between births.

"Vietnam implemented a program in the past to adjust and limit fertility, and this has led to a rapid population aging process."

While Vietnam had a one-child or two-children policy since the 1960s to check its then rapid population growth, the government is now encouraging families to have two children.