That's the point. None of the normal signs were there. Until she started to have labor pains, mistaking it for appendicitis, did she find out the actual cause of the pain. Whether you want to believe it or not is up to you, but it is a fact that this happened to her remains true. Seeing that there are 12 other siblings, of various ages at #13's birth (some even married with children of their own), someone would have slipped by now with the 'facts' if she had really known, yet she didn't.