And the full answer isn’t because of societal preferences – i.e. male babies being preferred over female babies for religious or economic reasons. Although that is a factor of course.
The main factor causing the imbalanced ratio of females to males is actually just evolution being clever (again). Due to the fact that men are more likely to suffer serious illnesses, take risks and be subjected to violence during their lifetime than women – coupled with the fact mothers are more likely to miscarry a male fetus during pregnancy – means that mothers will carry 150 male fetuses for every 100 female.
Take a look at this video by MinuteEarth explaining why there are more bouncing baby boys born every day than girls – even if the scales do end up balanced.