Stupid question, but are yours the same sex? If no, they are di-di.
I was in just the same situation - twins discovered at 19 weeks, same sex, placentas fused (and MZ twins, which of course we couldn't know at that point.) There are two things the techs look for: does the membrane meet the placenta at a "T" intersection or at a delta? A "T" suggests mono-di, a lambda suggests di-di. The other thing techs look for is a thick membrane. The chorion consists of the placenta and a membrane going all around the amniotic sac. Another MOT here described it as a water balloon in a zip lock bag. Mono-di = 2 water balloons in one ziplock = thin membrane between the babes, di-di = two zip lock bags next to each other = thick membrane.
Although I didn't understand it at the time, the tech did get excited to see my thick membrane. She pointed it out to me, and even my untrained eye could see it clearly. To me, that suggests di-di most of all.
So, how good is your u/s office? Skeptical though I am, I had a great MFM office. So I had to take their word that ours were di-di (although I do harbor some doubts.) Yes, the membranes thin out as the pregnancy progresses, but at 19 weeks a good tech should be able to see the difference. If it's a close call the doc has to assume mono-di, because of the need for more monitoring. So if they aren't worried, it's not a close call. You SHOULD be able to believe di-di.
If you have any doubt, get a second opinion and soon, before the membrane gets any thinner.
PS - Great news on that growth! Best wishes!
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