I also experienced this with my 2nd pregnancy, and figured it was no big deal, but my mother in law told me about her 4th pregnancy where she had the same thing. She saw her doc at 25 weeks and told him about it, and sure enough she was having pre-term labor. So, often it's harmless, but it is possible that something could be going on. With mine, I had many many internal exams once this started, as my OB wanted to be sure I wasn't dialating. At 30 weeks I was having so many, and couldn't stop them (6 in 40 minutes) that I visited L&D one evening and got put on meds to slow them down. For me, the only thing that really worked to lessen them was lying down, thus I ended up on bedrest with a young child to take care of. Please don't just ignore this. It can be your body telling you to slow down.
I ended up having my little guy at 37 weeks, so he did arrive a bit early. My mother in law was in the hospital on her left side for 2 months to keep her from laboring. I don't want to scare you, but I also don't want you to think of BH ctx as no big deal. They truly can be a big deal.