Quoting Amelia Margaret:" If it is short they will want to watch you super close to make sure you don't start to funnel and dilate. ... [snip!] ... call your OB right away. I ignored that feeling having no idea it could be due to my cervix and it turned out it was."
First paragraph pretty much sums it up. I have a shortened cervix and one doc wanted a cerclage but the new doc I had said I didn't need one. I ended up not getting one and was just fine the whole pregnancy. I was on pelvic rest (no sex) and could not excercise at all except for walking.