There has been a drastic decline in the child sex ratio (number of females per 1000 males between ages 0 and 4) in India and many of its states. This study aimed to examine if prenatal factors, such as change in sex ratio at birth, or postnatal factors, such as change in relative mortality of females and males, contribute to this more by analysing the dynamics of the child sex ratio. Changes in the child sex ratio during 2001–2011 were decomposed into a ‘fertility’ component attributable to prenatal sex selection and a ‘mortality’ component attributable to sex differentials in postnatal survival at the country as well as the state level. Between the prenatal factor and the postnatal factor, the contribution of the latter to the declining child sex ratio has been greater than the former in India as a whole and in most of the states. By focusing on both prenatal and postnatal factors, the imbalance in the child sex ratio in the country can be reduced to a large extent.