Maternity Pants Advice

Hey there, I'm 12w FTM and I bought some Gap maternity pants this week. I bought a size up because all the reviews said the pants beneath the panel were a little tight and didn't stretch. They fit well enough, but I wore them all day and I feel like I've been pulling them up all day. I know I'm going to get bigger, but I don't know how much bigger or how much room I'll need under the panel, any advice on whether to exchange for my regular size? They're return by mail only so I can't try on.