Had one with my first pregnancy 12hrs later baby was born. Went to have one with my second baby when the midwife started she said 'have you been having any pains at all' which I hadn't turns out I was already 4cm dilated. They don't work for everyone though. Good luck.
I had my first sweep at 40+1 where I was 2cm dilated but she stretched it to 3cm. I had my second sweep on 40+3 where I was 3cm but she stretched to 4cm. I went into labour that afternoon and she was born the next day. Good luck and I hope it works x