First, congratulations Lisa.
Second, I just have to say (with the risk of sounding annoying), please relax and don't stress too much about this. I know exactly what you're going through and I remember how stressful it was.
My girls were monochorionic/diamniotic. We knew that from early on--I think around 14 weeks or so, when I was assigned a perinatologist. My OB didn't think there was a membrane separating them at all, but the perinatologist found it (thankfully!). From the very beginning it was really hard to find the membrane every single time I had an ultrasound (and I had many).
The timeline is hazy to me, but I think I started going in for weekly NSTs at 28 weeks. At one of my NSTs the fluid on Baby B was low, so they kept me in the hospital for the day to monitor me, but it turned out to be a false alarm. I don't remember what the exact weight measurements were, but there was always about .25 to .5 lbs. difference between them.
I had a really awesome peri--she was very good but very cautious with me (I also had placenta previa, gestational diabetes, early contractions so I was in there a LOT
) and she decided around 30 weeks that she wanted me to come in for NSTs twice a week. When I was 32 weeks pregnant, Baby B (who had dropped down and was in the position of Baby A, but then never renamed them--talk about confusing! :LOL) was jumping on her cord. I was put in the hospital for good then & had to deliver them 6 days later due to her prolapsed cord.
Thankfully we dodged the TTTS bullet and I have two healthy, happy, non-stop 18 month old girls. I do remember their weights being quite a bit different from what they said they were going to be (again, the details are hazy--it was a crazy time). Baby #1 (but she was still Baby B) was 4 lbs. 3.8 oz. and 17 1/2 inches long. Baby #2 (Baby A) was 4 lbs. 12 oz. and 19 inches long. Baby #1 was always quite a bit paler than her sister when they were preemies, and some have said that that is an indication that some TTTS occurred, but I don't know that for certain.
Even now there's still about 1 lb. difference between them and one is about 1/2 inch taller than the other.
Good luck. Enjoy this time, seriously. I know it sounds kind of sick, but whenever I drive by the hospital where I delivered (& where my peri was), I kind of get wistful. I know it's not popular to say that heavily medicalized (is that a word?
) pregnancies can be a good thing around MDC, but in my case I'm absolutely convinced that it saved my daughters' lives.