Has anyone else's cervical length changed?

My cervix was measured at 4.6 at about 16 weeks.  I am 20 weeks now and it is measuring at 3.8.  My doctor told me to walk the results over to her, which I did, and she quickly looked at the measurement and said it looks good and rushed off.  I didn't get a chance to tell her that if she compared it to my last u/s she would have seen that my measurement had changed - and if this was a problem.  Anyways, I'm seeing her next week, so I will bring it up to her then, but in the meantime, I was wondering if anyone else has had this situation, and what the outcome was?  I'm worried about having an incompetent cervix, based on a mc I had as a teenager (I wasn't sure how far along I was, but pretty sure I was in my second trimester - I guess you could say I was in denial about being preggers).  

Thank you!