Up to 6 million females aborted over past decade, often when child was family's second and they already had a daughter

Families in India are increasingly aborting their second child if they know it to be a girl and they already have a daughter, a study shows.

Scientists estimate that up to 6 million girls have been aborted in India over the past decade by couples who do not want a large family and are determined to have a son. The practice is more widespread among wealthier and better educated Indian families, who are better able to afford the prenatal tests and medical intervention they want.

While it has been known that there has been a tendency to abort girls in India since the first census in 1871, the latest evidence suggests that the practice is common throughout the country.

The research, published in the Lancet, suggests that the Indian government's attempt to tackle the issue by outlawing ultrasound scans that identify the sex of a foetus has not worked.

The Pre-Natal Diagnostic Techniques Act was passed in 1996 to stop medical staff telling parents the sex of a foetus.

"It is unlikely that this act has been effective nationally because few health providers have been charged or convicted," write the authors, Prof Prabhat Jha from the University of Toronto and colleagues from India, including the former registrar-general of India, Dr Jayant Banthia. "We are not surprised by this lack of prosecution given that most primary care is with unregulated private providers."

This year's Indian census revealed that there are about 7.1 million fewer girls than boys under the age of six. The gap has grown substantially since the 2001 census, which found 6 million fewer girls, and the 1991 census, 4.2 million fewer.

The researchers used census data to estimate the absolute numbers of abortions for reasons of sex selection. They used information on more than 250,000 births in national surveys to work out the difference in the girl-boy ratio in second births in families where the first child was a girl.

They found that the ratio in second births where the first child was a daughter fell from 906 girls per 1,000 boys in 1990 to 836 in 2005 – a drop of 0.52% a year. But there was no decline in the ratio among couples whose first child was a son.

The authors estimate that between 3 million and 6 million girls were aborted from 2000 to 2010. Over the 30 years from 1980 to 2010, there could have been as many as 12 million abortions of girls.

In a commentary, two leading experts point out that the desire to have a son appears to influence the behaviour of expatriate Indians too. Higher ratios of sons to daughters in second births, when the first child was a girl, have been found among Indians living in the US.