The international ratio at birth is about 105 boys for every 100 girls, but in Vietnam -- in an echoe of trends in China and India -- the imbalance has grown to 110-100 and is as high as 120-100 in some provinces.
The "missing daughters" phenomenon, experts fear, may worsen in the current lunar Year of the Golden Pig, deemed a very auspicious time to have a son in Vietnam, a traditionally Confucian country.
In the mid-1950s socialist North Vietnam passed family laws promoting gender equality, but old habits die hard. Posters of naked baby boys, and dolls, remain popular in markets and attest to a continued preference for boys.
Vietnam for decades had an official two-child policy, and although this was never as rigorously enforced as China's one-child policy, small families are still promoted by party officials in districts, villages and workplaces.
Although Vietnam in 2003 banned fetal sex selection, many doctors tell parents-to-be if they are expecting a boy or girl at ultrasound practices that have mushroomed in increasingly affluent Vietnam.
Demographers have long suspected a gender imbalance at birth in Vietnam, where abortion has been legal for decades and is widespread, but they have so far lacked reliable nationwide data to back up their fear.
Now a UN Population Fund report, using the latest census data released by the communist government, has set off alarm bells, highlighting a "growing concern that the sex ratio at birth is becoming unbalanced in Vietnam".
"Reasons for this include pressure to adhere to the two-child policy coupled with a preference for sons and the ready availability of ultrasound and abortion," said the report, adding that the highest provincial ratio was 123 boys to 100 girls.
"In some locations the preference for a son is simply stronger than others, and with high-tech ultrasound, sex can be detected at an early stage and a female fetus can be aborted," it said.
Anecdotal evidence has long suggested a problem in Vietnam, where many kindergardens now appear to have more boys than girls.
Men in Vietnam have traditionally carried on the family lineage, inherited homes and land, and cared for elderly parents as well as overseeing funerals and ancestor worship rituals.
One Hanoi woman, Tran Hanh Linh, 30, says she remembers what her mother-in-law said when she moved into her husband's family home.
"She told anyone visiting our family that if I delivered a baby boy, she would treat me like a queen," said Linh. "I wondered what would happen if my child was a girl. Would she kick me out?"
Linh got lucky, saving the family peace by having a boy two years ago.
But another woman, 33-year-old Thao, was devastated after recently finding out that, having already borne one daughter, she would now have female twins.
"My parents-in-law are from a very conservative generation," she said. "I must say I suffered a lot of pressure, from them and even from my husband. He was obsessed with having a son, maybe to prove his manliness.
"My parents-in-law want us to have a boy because they are really afraid that after they die, no one will maintain the family bloodline and no one will pray for them on their death anniversaries."
Thao, who did not give her full name, said she earlier had an abortion after a doctor and two fortune-tellers told her she would have a girl. She went ahead with the latest pregnancy after being told she would have twin boys.
"We were really happy then," she recalled. "But two months later, when the doctors could observe the babies better, they said I would have twin girls instead. We were really sad. My husband was in shock."
Daniele Belanger, a Canadian expert who studies the trend in Vietnam, said: "Women suffer a great deal from the clash between the high demand for sons and the low demand for children.
"Sex selective abortion has the power to free them from such contradictory internal desires and external pressures," Belanger, Canada research chair and director of the Population Studies Centre at the University of Western Ontario, wrote in a recent research paper.
But, while empowering women in the short term, she warned, sex-selective abortions would most likely "further threaten their position in the long term".
"Once this imbalance reaches marriageable age groups, a shortage of women poses serious problems, particularly in societies where marriage is a nearly universal norm," she wrote.
A similar "marriage squeeze" has hit one-child China. In South Korea and Taiwan it has been a factor driving bachelors to seek wives in other countries, including Vietnam, through illegal but ubiquitous marriage brokers.
A recent poll by the Korea Institute for Health and Social Affairs showed the preference is on the decline in South Korea, with only 10.2 percent feeling they must give birth to a son. The decline was attributed in part to higher education levels.
The UN report warned that "Vietnam needs to act now if it is to avoid the situation of more men than women evident elsewhere in Asia".