Cervix during pregnancy

I have already submitted this question to my OB.

However, I would like to ask the community at large what their experiences have been.

At my 17 week ultra sound my cervix measure 46mm. I just had a 21 week ultrasound and it is now measuring 34.6mm. I am concerned that it is shortening. I understand that it’s not a cause for concern until 25mm. I am wondering if anyone had their cervix shorten through pregnancy and if you went into preterm labor.