Needing bump photo advice?! ?

This is for my baby bump photos that I want to do to document the progression of my pregnancy. I am shopping for a long stretchy dress to use throughout the 9 months of carrying my little bun in the oven for the photos to show the change. I've never been pregnant so it's very hard to estimate how big my belly will get or if I will outgrow a stretchy dress but I definitely want to make sure it fits and am sure I can push it down at first over my belly to show the bump when it is to big! But I'm scared to get one way to small or big.  How many sizes up do most do for their bump photo clothes? What's your experience when you were pregnant? The stretchiness should help but I'm guessing I should get it one that's currently a little big. I'm probably about a size 9 in pants (though at 10 weeks I've forgone jeans as they dig into my belly and  cause reflux) and medium in shirts if that helps. Thanks so much! <3