I get 2 different ones- there's a scratching type one and a sharp knife pain one. Anyways, for me it IS effacing and dilation. I started feeling it about 32 wks this time and at 33 wks was 2 cm and 50%.
I get those too. I heard (here on MDC, I think) that those were the baby jabbing its elbow or some other hard bit into your cervix, so that's what I've been picturing when it happens. I know I started getting them before I found out from the OB that I was mostly effaced and a tiny bit dilated, but I couldn't say whether the two things are related or not.