mine were like a tightening sensation in my lower back (i had a posterior bub) and like Nelle said came in waves. For the last few weeks of my pregnancy i'd been get all having sorts of odd niggly pains so when I first felt that back pain my inital reaction was "oh that's a new one, let's see if it happens again then i'll know it's a contraction." Sure enough I had another 15mins later and they were in full swing from then on in!