Cerclage

So I was informed on Tuesday at 20 weeks pregnant that my cervix is an abnormal length. It was 1.5cm on Tuesday and they started me on Progesterone. I went back in on Thursday and it was a shocking .5cm! They checked for dilation and thankfully there is none my cervix is very closed. It is most likely due to the Progesterone. Baby is super active and healthy but if I were to dilate that would definitely put them in danger. They’ve mentioned a cerclage but won’t do one yet bc my cervix is not opening. I am on Tricare so I’m going to a military hospital so second opinions and all that are very limited. I wanted to know if any of you moms have gotten a cerclage and what the criteria was for you to get one for your doctor?