I've been able to feel them since quite early in my pregnancy and get them quite frequently. I think maybe about 18 weeks was when I started feeling them just as a tightening sensation in my tummy for a few seconds, and as my belly has gotten bigger they have gotten more noticable. Most of the time they just feel like a strong tightening in my belly and it goes rock hard, but not actually painful. I did have two nights in a row at 30-something weeks where they went on for hours and were actually quite painful and I wondered after a while if I might be in labour LOL, but that hasn't happened again since. Only the non-painful kind thank goodness!