Sex-ratio is in favour of females because

Girl babies have greater resistance to disease 

The sex ratio refers to the number of females per 1000 males in a given area at a specified time period. Historically, all over the world it has been found that there are slightly more females than males in most countries. This is despite the fact that, slightly more male babies are born than female ones; nature seems to produce roughly 943 to 952 female babies for every 1000 males. If despite this fact the sex ratio is somewhat in favour of females, this seems to be due to two reasons. First, girl babies appear to have an advantage over boy babies in terms of resistance to disease in infancy. At the other end of the life cycle, women have tended to outlive men in most societies, so that there are more older women than men. The combination of these two factors leads to a sex ratio of roughly 1050 females per 1000 males in most contexts.