Up to 10 million female fetuses may have been aborted in India over the past two decades following prenatal gender checks, according to a study published in a leading British medical journal.

In a country where many regard daughters as a liability, researchers found second children were less likely to be girls if the firstborn child was a girl. They also found 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.

Daughters traditionally belong to future husbands' families in India, which maintains the custom of dowry.

According to the report published in the Lancet on Monday, the researchers studied data on female fertility from a continuing Indian national survey, analyzing information on 133,738 births.

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 1997 in India. 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," it said.

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

In India, fetal sex determination and medical termination of pregnancy on the basis of a fetus' gender have been illegal since 1994.

"However, there was ample published evidence of rampant sex determination and female feticide," it said.

The preference for boys has skewed the gender ratio in India, a nation of more than 1 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 census-takers.