Originally Posted by
Akktim
Mystique pretends to be Moira and gets knocked up by Xavier. She time travels to some unknown point in the timeline for the duration of the pregnancy and returns to "last year" and gives birth to Xavier Jr. Baldy Junior grows up without any knowledge of his true parentage, and his mutant abilities manifest at some point in the future.
Or, Mystique-as-Moira's and Xavier's copulation took place several years ago, and Mystique time traveled to "last year" to give birth (because reasons). And that's why the O5 can't return to their original place in time.