This paper presents the first estimates of the causal effect of facilities for prenatal sex diagnosis on the sex ratio at birth in India. It conducts a triple difference analysis across cohort, birth order and sex of previous births. Treated births are those that occur after prenatal sex detection becomes available at birth order two or more in families that have not yet had their desired number of sons (or daughters). The three implied control groups are births that occur pre-ultrasound, births of first order and births that occur after the family has achieved its desired sex mix of births. We identify a significant divergence between the treated and control groups. We consider alternative hypotheses and conduct an array of robustness checks to show that the divergence of the sex ratio of the treated group from the normal biological range that characterizes the control groups is on account of female foeticide. We estimate that as many as 0.48 million girls p.a. were selectively aborted during 1995-2005, which is more than the number of girls born in the UK each year. The estimates suggest that Indian families desire two boys and a girl; previous studies often assume that the desire is for at least one boy. The incentive to conduct sex selection is increasing in birth order and family socioeconomic status, both consistent with stronger incentives to sex-select as fertility approaches its target.