The Lancet actually addresses the issue:

According to research published by the British medical journal, Lancet, selective abortions and a traditional preference for male sons could be among the causes of this serious phenomenon. A team of scientists, who analyzed data on fertility on the basis of a nationwide survey in India with six million respondents, discovered that in 1997, half a million less females were born than expected. Projected over a 20-year period, the figure climbs to 10 million.

Experts said resorting to selective abortions of female fetuses was the mostly likely explanation for the phenomenon. “We conservatively estimate that prenatal sex determination and selective abortion accounts for 0.5 million missing girls yearly," said Prabhat Jha of the University of Toronto, who led the research team. "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."