I've poked around some on the TTTS boards, just because this pg is exactly the sort where there's a risk of TTTS. (It only occurs in identical twins that share a placenta, which is a small minority of all twins, and even of identicals.)
If there's been a change in the relative sizes -- if they were measuring closer together earlier, then I think I would insist on being seen more often. I'm at 20.5wks now, and since 15wks, I've had an ultrasound probably no less frequently than every 10 days. (This next one will be almost 2 weeks, but that was complicated by my going out of town for about 10 days...)
As of the last time for us, both boys were measuring almost a week ahead of schedule, and one was 1d bigger than the other. They've consistently measured that way, though, since 12wk. If this relative rate changes, I'll be concerned -- but I already know that my ob and perinatologist (I'm bouncing between them for appointments, and both do ultrasounds every time) are watching for it.
The main thing about TTTS is that when it does happen, it goes downhill FAST. (Faster than 2.5 wks.) Many docs "prescribe" bedrest and at least 3 protein shakes (Boost/Ensure) a day on top of the diet you already had. The TTTS Foundation site has good info about the options, and there are even threads with doc recommendations in the forums. (Some options are dependent on having a doctor who can perform the procedure nearby.)
mommy to three boys: A, 3/04
and identical twins B and D, arrived 9/08