More than 10m female births in India may have been lost to abortion and sex selection in the past 20 years, according to medical research.
Indian cultural tradition favours boys
Researchers in India and Canada for the Lancet journal said prenatal selection and selective abortion was causing the loss of 500,000 girls a year.
Their research was based on a national survey of 1.1m households in 1998.
The researchers said the "girl deficit" was more common among educated women but did not vary according to religion.
The unusual gender balance in India has been known about for some time.
In most countries, women slightly outnumber men, but separate research for the year 2001 showed that for every 1,000 male babies born in India, there were just 933 girls.
The latest research is by Prabhat Jha of St Michael's Hospital at the University of Toronto, Canada, and Rajesh Kumar of the Postgraduate Institute of Medical Research in Chandigarh, India.
They found that there was an increasing tendency to select boys when previous children had been girls.
The sex ratio is so skewed in some states, men cannot find brides
In cases where the preceding child was a girl, the ratio of girls to boys in the next birth was 759 to 1,000.
This fell even further when the two preceding children were both girls. Then the ratio for the third child born was just 719 girls to 1,000 boys.
However, for a child following the birth of a male child, the gender ratio was roughly equal.
Prabhat Jha said conservative estimates in the research suggested half a million girls were being lost each year.
"If this practice has been common for most of the past two decades since access to ultrasound became widespread, then a figure of 10m missing female births would not be unreasonable."
Sex selective abortions have been banned in India for more than a decade.
Experts in India say female foeticide is mostly linked to socio-economic factors.
It is an idea that many say carries over from the time India was a predominantly agrarian society where boys were considered an extra pair of hands on the farm.
The girl child has traditionally been considered inferior and a liability - a bride's dowry can cripple a poor family financially.
The BBC's Jill McGivering says the problem is complicated by advances in technology. Ultrasound machines must be officially registered but many are now so light and portable, they are hard to monitor.
Although doctors in India must not tell couples the sex of a foetus, in practice, some just use coded signals instead, our correspondent says.
Last year the well-known religious leader and social activist, Swami Agnivesh, began a campaign across five northern and western states against female foeticide.
"There's no other form of violence that's more painful, more abhorrent, more shameful," he said.