Increased access to technology that allows parents to know the sex of their foetus has left Asia short of 117 million women, mostly in China and India, the UN said on Thursday.

The trend is expected to influence the affected countries for more than 50 years, particularly through a shortage of brides for Chinese and Indian men, according to experts at a conference organised by the UN and Vietnam in Hanoi.

"This skewed population sex ratio reflects a preference for sons, in combination with increasing access to new sex-selection technology" such as ultrasound, the UN Population Fund said in a conference paper.

Sex determination leads many parents to resort to "selective abortions", said French demographer Christophe Guilmoto.

In most countries the sex ratio at birth ranges from 104 to 106 male births for every 100 females, "but its level has gradually increased over the last 25 years in several Asian countries, particularly in China and India", the UN said.

UN figures covering the past few years show China with 118.1 male babies for 100 females, India 110.6, Azerbaijan 117.6 and Vietnam 111.2.

"Meanwhile, postnatal discrimination -- expressed through excess deaths among female infants and young girls -- has not entirely disappeared from several countries and reflects the relative neglect of female children," Guilmoto said in a conference paper.

The UN says a preference for sons reflects socio-economic influences and deep-rooted traditions in which sons alone inherit property, care for ageing parents and so on, while daughters may require dowries and leave their families once married.

Guilmoto said that even if the sex ratio at birth returned to normal within 10 years, Chinese and Indian men would still face a "marriage squeeze" for several decades.

"Not only would these men have to marry significantly older, but this growing marriage imbalance would also lead to a rapid rise in male bachelorhood... an important change in countries where almost everyone used to get married," Guilmoto said.

In China most parents are banned from having more than one child.

"Dealing with the future demographic consequences of past and present sex imbalances at birth and their societal impact may soon become the next challenge to respective governments," Guilmoto said.

Though some countries like Vietnam prohibit foetal sex determination, Guilmoto said such bans are difficult to enforce.

Heeran Chun, of Jungwon University in South Korea, said her country is "unique" in bringing down to near-normal levels its high sex ratio which peaked in the early 1990s.