No warning for DS1, 21 years ago now I went for my 39 week check the doctor did a VE (it was 'routine' back then) She said I was already about 1.5cm dilated then she said oops! Swore she didn't do it on purpose and as she (TMI) removed her fingers it was Niagara falls over the side of the exam bed.

DS2 I had a show the day before then started with a slow leak the next afternoon.

DS3 No warning got up and went for my morning wee and as I was sitting there I heard a loud POP! Felt it too and that was it, not quite Niagera falls again but then soaked a few pads before tapering off a bit.