it actually just depends on your body. some women dilate really early on in their 20th week. I'm 39+3 and not dilated or effaced.

with my daughter I delivered her at 40+1 I wasn't dilated at all but I was 100% effaced for like 2 weeks. after 72 hours of contractions 3 minutes apart I finally started to dilate and delivered her 14 hours later.