Cervix shortening after cerclage

I had an emergency cerclage at 15 week. After one week my cervix measured 4.6cms. In the 3rd week post cereal age it measured 3.5cms and now in 4th week post cerclage during anatomy scan it measured 3.5cms, few parameters were still left to be checked after the baby changes the position so they did Sonoghraphy again after 3-4hours and before that I had used washroom, now my cervix measured 2.9cms. My Dr said that it can happen since the bladder might be full earlier. But I am concerned why the length keeps reducing every week, has anyone experienced the same and have success stories ?