Cervical Shrinkage Questions

Hi all! First off, I'm 18 weeks with di/di twins and this is my first pregnancy. I have had an ultrasound every two weeks since week 14. Week 14, my cervix measured 5.1cm and week 16 it measured 3.1cm. Have been feeling a lot of pressure. Had another ultrasound this week to keep an eye on it but have not heard the results, have another appointment in two weeks. The doctor seemed very concerned and mentioned cervical stitching or a gel that I may have to apply. I have read a lot online about bedrest in this sort of situation. Anyone else going through this? Were you put on bedrest? Did your cervix stop thinning? I'm just worried and want to plan but I know it's out of my hands.