Shortening cervix. How worried should I be?

I’m pregnant for the 6th time after 5 consecutive losses. 18 weeks today, which is the farthest I’ve ever gotten. Last night, I ended up in the L&D ER bc I lost my mucus plug—like 3-4 inches of it, which I understand could be all of it. 😭

Anyway, they did a transvaginal ultrasound (and a physical exam, in which nothing looked concerning) and my cervix measured 2.9 cm. Exactly 2 weeks ago at 15w6, it measured 3.9 cm.

I know 2.9 is technically “acceptable,” but it seems like it shortened SO quickly in just 2 weeks. How worried should I be? 🥺

For what it’s worth, I was discharged with instructions to call my OB as soon as he opened this morning, which I did and had to leave a message. He has been great through my losses, so I do think I’m in good hands with him. I’m just curious if this quickly changing measurement points to incompetent cervix or if it could just be a variation of normal.