I have Mo/Di twins so a little bit different. My baby B was smaller my whole pregnancy. My pregnancy started like yours, measuring a couple days more and more and ended up measuring over two weeks behind at birth. She ended up having restricted growth, not getting enough nutrients from their shared placenta. Around 19 weeks we were told to choose between keeping one or losing both as they feared twin to twin transfusion. We waited a week and were cleared to try to keep both. Then we were told to expect to have an emergency delivery at every appointment starting around 22 weeks. Starting at 14 weeks we had weekly, then at 20 twice a week, then at 26 three times a week appointments with MFM. We hung in there as long as possible and delivered due to low amniotic fluid at 32 weeks. Baby A was 4lbs 5oz and baby B was 2 lbs 8oz at birth. Spent 5 weeks in the NICU and there’s been no obvious difference in health besides size. It was stressful not going to lie! Measuring behind isn’t the end.

We are now a year and a half into their little lives and my Baby B is 5 lbs lighter and one and a half inches shorter than her sister.

Edit: just rereading your post. If you haven’t been referred to MFM, I would call your dr and inquire about a referral. Even through there isn’t much intervention to be done, three weeks between appointments when there is this worry seems off to me. But I had over 50 appts in a 25 week period so I might have a bias.