Around 20 weeks for me as well, back then they were mainly tightenings now they can get really tight and a bit uncomfy at times, they last anything up to 2mins.
With Poppy whne i went for my 20 week US the sonographer pointed out that I was having them during the scan you could see the uterus contracting down onto her and then she would kick and push against it.ouch!! theres lots going in there!!!