Everyone is right about the dearth of options for our outsized bellies. BUT, I have found a pretty good alternative to complete tent-shirts that I strongly recommend. Buy or make some "maternity belly bands". There are some on the market that are very overpriced (like Bella Band or baby be mine maternity bands). Or if you sew, you can make your own. Here is a tutorial, not for pg women, but you could adapt to your size: http://www.planetjune.com/blog/haram...rmer-tutorial/
. Note that in Japan, these belly bands are called "haramaki" and are a trend there, also you can find links about them on the net under "Japanese Belly Warmer". In Japan there is a belief about keeping belly warm, especially in pregnancy.
Anyway, what I actually did that has worked pretty well is to take some old stretch-cotton t-shirts (ones that have 5% or so Lycra in them) that were "pitted out", and just cut them off right below the pits. Then I wear these over my tummy pulled up above my belly button and pulled way down to cover my low-rise maternity pants or sweats (as well as the thick maternity band that goes around the waist band of my maternity pants). I have different colors, brown, pink, green and they nicely compliment some of my tops that I wear over them. Fortunately, I had a nice selection of these t-shirts around that were just about to hit the "give away" or rag pile. But you could also buy them at a 2nd hand store or get some inexpensive ones at a Marshall's, Ross, Target, or what have you.
I am 30 weeks and measuring 40 cm and my normal old t-shirt size has worked fine for this purpose (thus far no need to use a bigger t-shirt size for the "belly band" AKA cut-off-T-shirt).
This allows you to have your belly covered, look actually pretty fashionable (since little undershirts are "in"), and "grow" with your favorite maternity shirts without having to set them aside because your bare belly peeks out below them.
The other advantage is that these bands help cover the "plumbers crack" that can be so frustrating with the low-rise jeans. I really love this solution, because I feel so much comfier and more covered up and not like I'm hanging out all over the place.
The only disadvantage I have found to the cut-off t-shirt method is that the non hemmed part where you cut the t-shirt rolls a little bit so that you can see the line of it under your shirt if your top shirt is tight. But I prefer this to having my belly hanging out below.
Okay, can you tell I have been pretty happy with this little trick? I'll stop rambling now. Hope someone finds this useful.