I can read, thank you. Please Register or Log in to view the hidden image! But fetuses still get their oxygen from blood. The reflex is there, the environment is not yet. Because the brain is still less active than when fully woken up and it needs a kick. (the reaction is slow, awarness of surroundings less than good) When we wake up there's a whole new heap of external stimuli for the brain to analyse, much more than when we were asleep, therefore it suddenly needs more oxygen.