Yeah, it's stories like yours (and every other twin mom I have EVER talked to or read about) that make me a nervous wreck! I have very highly recommended OB, and yet they are not doing ANYTHING different until at least 24 weeks. We had the u/s to confirm (and "count heads"!) at the RE at just over 6 weeks, then another to confirm that both twins were still there, growing appropriately etc at 8.5 weeks before graduating to the OB. There, they have done NOTHING for me, even after having difficulty hearing one heartbeat at least every other time (one is behind the other, still, as far as I know). It's so frustrating! So we'll be going from 8 weeks to just about 20 without so much as having confirmation that both are growing appropriately. The OBs seem to take complete consolation in the fact that my uterus is 3, 4 and then 5 weeks ahead (despite little to no weight gain). I'm sure that does count for something, but they are certainly making my pregnancy MORE stressful than it would be already, instead of less!
ETA: We tried seeing a peri as well, just for some more proactive following since it was so tough getting this far that we don't want anything to happen now (not that anyone ever does!). Unfortunately, becuase DH and I are both young and I have no significant medical anything (we just had azoospermia, I checked out fine) they won't see me. I suppos that should be comforting as well, but it's not!