About 10 million female fetuses may have been aborted in India over the past two decades, according to research published on Monday.

A team of scientists who analyzed female fertility figures from a national survey of 6 million people in India found that there were about half a million fewer girls born in the country in 1997 than expected.

Extrapolated over 20 years, the figure would be 10 million, the researchers reported in a study published online by The Lancet medical journal.

They added that selective abortion of female fetuses is the most plausible explanation for the skewed sex ratio.

“We conservatively estimate that prenatal sex determination and selective abortion account for 0.5 million missing girls yearly,” said Dr. Prabhat Jha, of the University of Toronto in Canada, who headed the research team.

“If this practice has been common for most of the past two decades since access to ultrasound become widespread, then a figure of 10 million missing female births would not be unreasonable,” he added in a statement.

The findings support estimates by the Indian Medical Association, which has said that five million female fetuses are killed in India each year.

Jha and his team found that the sex of the previous child was a determining factor in whether a female fetus was aborted. Fewer females are born as second and third children if the first child in a family is a girl.

The researchers said lack of girls as a second or third child is more pronounced in educated, rather than illiterate, women.

“To have a daughter is socially and emotionally accepted if there is a son, but a daughter’s arrival is often unwelcome if the couple already have a daughter,” Professor Shirish Sheth, of the Breach Candy Hospital in Mumbai, India, said in a commentary on the report.

Fetal sex determination and medical termination of pregnancy based on the sex of the fetus have been illegal since 1994, but Sheth said there is published evidence of rampant female feticide in India where daughters are regarded as a liability.

“Female infanticide of the past is refined and honed to a fine skill in this modern guise,” Sheth added.

Dr. Rajesh Kumar, of the School of Public Health in Chandigarh in India and a co-author of the report, said missing females is a growing problem.

“Our study emphasizes the need for routine, reliable and long-term measurement of births and deaths,” Kumar added.