Fewer daughters have been born to couples who have not yet had a boy, according to the study's report published in the Lancet on Monday.

The journal said its researchers studied data on female fertility from a continuing Indian national survey of six million people in 1.1 million households.

Selective abortion

Analysing information about 133,738 births, the researchers found that couples were less likely to have a girl as a second child if their first child was a girl and that the deficit in the number of girls born as a second child was more than twice as great among educated mothers than among illiterate ones.

Based on the natural gender ratio from other countries, they estimated that 13.6 million to 13.8 million girls should have been born in India in 1997.

"We conservatively estimate that prenatal sex determination and selective abortion accounts for 0.5 million missing girls yearly"

The Lancet

However, 13.1 million were reported, the study said.

"We conservatively estimate that prenatal sex determination and selective abortion accounts for 0.5 million missing girls yearly," the study said.

"If this practice has been common for most of the past two decades since access to ultrasound became widespread, then a figure of 10 million missing female births would not be unreasonable."

Ultrasound, used to check a foetus's health, can also find out its gender.

In India, foetal sex determination and medical termination of pregnancy on the basis of gender have been illegal since 1994 . "However, there was ample published evidence of rampant sex determination and female feticide," the report said.

liability

The preference for boys has skewed the gender ratio in India, a nation of more than 1.06 billion people. The number of girls per 1,000 boys declined in the country from 945 in 1991 to 927 in 2001, according to government figures.

Many people in India regard daughters as a liability because they traditionally belong to future husbands' families. The custom of dowry prevails in India, and many families borrow money for gifts to the husband's family.