Registering exact time of birth

until the baby starts to exist on the prana of the world instead of the prana of the mother they are not born, that happens at first breath. At that same moment, the heart valve is designed to close. Generally a baby screams as soon as it gets air, that's not always the case, but its important to watch for the breath, but not always easy to see that. 

Thanks Ernst! This is the most satisfactory answer I have received on this point. Is there a textual source for this? 

no there is not.