I had no build-up of contractions with my spontaneous birth at all!
My very first contraction was at 6.50am - no warning, niggles, nothing. Got up as it was incredibly painful. Had a shower and rang my brother (looking after kids). My contractions immediately came at every 2 minutes and I was clinging onto the door frame of the bathroom.
My brother lives 30 minutes away, so rang him again to hurry up, and again on his mobile in the car. He arrived here at 7.35am - me and DH walked out the door as he walked up the path. Straight into the car for the 5 minutes drive to hospital.
Into the maternity unit at 7.45am - midwife knew delivery was imminent and kept saying "wait for me, don't push", whilst trying to get her gloves on. Literally got my trousers off, she examined me (fully dilated), my waters broke and DD2 literally shot out at 7.52am!