At the moment of conception, the sex ratio in humans is 1 to 1. Yet more boys than girls are born: ~105 males for every 100 females.
Girls develop more quickly in the womb but are also more likely to die there, at least in the first few months. Out of the womb, boys are more prone to death throughout life, tending the ratio to return to 1 to 1.
Boys face a triple whammy. They are more likely to be born preterm, and if they are, they have a greater risk of death, disability or blindness. And even when they are full term, they have a higher risk of birth complications. ~ English physician Joy Law
While sex ratio skewed toward males is common in several animal species besides humans, researchers cannot account for this balancing, as no feedback cycle is apparent.