im nearing the end of my second tri at 24weeks, and ive been having them since as early as 19weeks. I didnt know it, but my CNM felt it while measuring my belly (got tight, then relaxed) and then again last week while having a sono I was having them. When I asked how you can tell I was told my placenta had a "hooked" shape due to the contractions. I think most people have them, you just dont feel them!