Mine started weeks ago, probably around 24 weeks???? I get this weird tightening sensation and sure enough when I put my hand on the belly, it's rock hard. Then about 10 to 20 seconds later, the stomach is normal again. Sometimes I get 5 or more BH a day, but spread over the day. Once I freaked out as I thought I had more than 4 in an hour, and they say if that's the case to ring the doctor/midwife in case it's pre term labour.