Prior pregnancy w/ twins. Had bi-weekly monitoring starting at 16 weeks. First measurement was 2.8. Loss was at 20 weeks. But I caution you, I had A LOT of other issues.
Current pregnancy w/ singleton. Currently 16 weeks. Had cerclage placed at 13 weeks. Was less then 3 and 1 cm dilated at time of surgery (a week prior to this, u/s measurement said I was long and closed). I have had two other measurements at 14 weeks and 16 weeks. At 14 weeks, cervix was 3. At 16 weeks, cervix was 4.5.
Other issues, repaired septate uterus and 6 surgeries w/ dilation. Also, during one surgery, RE tested cervix and found it easy to dilate. Those the placement of cerclage during this pregnancy.