I had this done twice. Every time I had it done I would get contractions and the first time I lost my mucus plug right after but it just went away. Now I am 40 +1day. My doc was going on Vacation so he had done it at 38 weeks 2cm 70% effaced and 39 weeks 3cm 90% was the second time. Now, nothing is going on no contractions, no pressure nothing.
I will be 40 weeks on Friday and will ask for this to be done also, hopefully it helps I am only 2cm dilated and 50% effaced as of last week. Good luck to you!