why do babies start crying a moment after the parturition?

Some babies come out practically wailing, but most will take a minute. The cry is a good indicator of how the baby is breathing. If a baby has no evidence of mucus and looks pink with good reflex there is no reason to make the baby cry. Some babies are quite alert at birth and will look around and take obvious clear breaths without crying. Since too few babies are born today without medication we have come to expect that the baby will be born tired and needing some help with breathing. This makes that cry all-important.

The babies lungs are filled with mucus that is squeezed out as the baby goes through the birth canal. If not all of this gets out the baby can have issues breathing, the crying helps make sure that they are breathing with no difficulties and that no mucus remains. Also, if born by C/Section then they have to suction the mouth and lungs because the mucus never had the chance to be squeezed out.