21 week sonogram and cervix?

I had my cervix measured at 13 weeks and it was 4.5 (anything over 3.5 is good from what my dr says). I had it measured today at 21 1/2 weeks and it is 3.9. My dr still says that it is okay and I am at a low risk for preterm delivery. I am still a little concerned as I'm worried that it will be even less, get below 3.5 as the pregnancy progresses. Any have any insight?