I've had them since 16 weeks. My dr said I shouldn't be having them that early like I was. I was contracting on her monitor. She's watched me close since then. I'm 26 weeks now. Over this past weekend, I was having lots and shebeens me to labor and delivery to be checked. I know some doctors don't worry too much about it, but mine likes to jeep a close eye. I've had preterm labor before though...
It's worth mentioning to your doctor/midwife. They may want to make sure it's not changing your cervix. Mine was in the low end of normal when it all started...