I got these white Gloria Vanderbilt jeans (which, can I just say that I am loving the fact that these are GV's? Makes me feel so Dynasty!) and just added a pregnancy panel. I didn't do anything special, just cut out enough room for The Belly and added about 1/3 yard of some nude jersey, and then cased some elastic on the top. I also cropped them and made them more like cigarette pants because I figured I may as well go for it with these white jeans. I'm already breaking rules wearing white after Labor Day! I think they're so much cuter at this length.
I also did a basic pregnancy panel on these three pencil skirts, purchased for $1 a piece at a local thrift shop.
I didn't follow any tutorials for these panels, I just winged it. Basically, as long as you sew the jersey under your belly and add some elastic to the top to hold the garment in place, you're pretty good to go. I'd also use a stretchier jersey (sorry, I don't know many technical terms with jersey) as you'll grooooooooooooow.
I did find out today that JoAnns has pregnancy panels, which I never knew, so that may help out too. I have no idea what they look like, the guy at the cutting table just told me about them. I'm sure I'll be repurposing some more pants & skirts soon, so I can do a tutorial if anyone's interested.
xoxo,

