When minister of women and child development Maneka Gandhi suggested registering the sex of every child in the womb to ensure the girl child is not killed, she probably did not understand the complex calculus that Indian would-be parents go through -when to have a child, how many, and boy or girl. A glimpse into this intriguing decision-making process undertaken by about 2.5 crore parents every year is provided by Census 2011data.Among women who had only one child, the sex ratio was just 782 girls to every 1,000 boys born.This is much lower than the overall sex ratio of the population, (943 females for every 1,000 males), and even lower than the sex ratio for children aged up to six years (919). But it is not yet the rock bottom. Among families with two children, the ratio plummets to 720 girls. This is where the sharpest drop has taken place, from 742 in 2001. However, with three children, it improves to 814.In families with four children, it improves further to 944 and by five, it reaches 1,005 girls for 1,000 boys. Census data indicates that the sex ratio is poor when women have one or two children, but gets better as they have more children. Two factors are at play here, explains population expert Purushottam M Kulkarni, till recently professor at JNU. One is sex-selective abortions and the other is sex-selective "stopping practices", which is stopping having children based on sex of those born."Those women who stop childbearing if the first one or two births are sons remain in the category of 1-2 children and those who go on for another child because the first two or three births are daughters get into the category of 3 or more children (that is, there is selectivity). As a result, the sex ratio of children of women with one or two children becomes highly masculine and, for the children of women with 3 or more children, less masculine," he told TOI.For women with only one or two children, the sex ratios have fallen since 2001, substantially so in the case of mothers of two. This, says Kulkarni, indicates a strengthening of the mentality of son preference that is driving the whole calculus.Sex-selective abortions, with all the legal fencing, are still prevalent and may be used to determine the first and especially second births. The overall sex ratio is primarily affected by such illegal practices and not by sex selective stopping. The Census data also gives the number of surviving children for each type of family. This throws up an unexpected finding ­ the gap between survival rates of boys and girls is closing.