I decided these couldn't be that hard to make, given the right materials, and I had been wanting to make a few maternity things. One maxi skirt and one pair of lace shorts later, I think I've got it!
You need: *1/2 yard (or a little more depending on your size) of "Poly Interlock." I got mine at Hancock's, Joann's probably has it too. Mine is neude, sorry it blends in a bit with the carpet!
*A garment that you made or are repurposing
*A maternity garment to use as a template (although it wouldn't be too hard to just look at these pictures and take a few measurements on your own body, just remember the belly panel needs to fit snugly to keep your pants on comfortably)
*Sewing machine, matching thread, etc...
1) Lay your piece of poly interlock on a flat surface, fold it so that the grain runs up and down and you have two layers with a fold on one side. Now fold your templace garment in half as shown, that puts your seam in the back and a fold in the front (matching the fold of the poly interlock). Using the existing belly panel as a template, cut around the top and back, then along the curve of the bottom, leaving room for seam and hem allowance on all sides (don't cut the fold!).
2) Now that you have your pattern piece cut out, fold right sides together and sew along the back. Be sure to set your machine to a stretch or zig zag stitch, it's also helpful to use a stretch needle and increase your stitch length to about 3.0. Turn right sides out, fold the raw edges to one side and topstitch. It looks more professional and adds a little durability!
3) Now, hem the top edge of the belly panel. Remember the curved edge fits under your belly, the top edge is straight. There's no use trying to iron and pin this stuff, it's so slinky. Just turn under about 1/4", then another 3/4" or so as you sew. This seam is against the grain (the stretchiest part of this type of fabric) so it's essential to keep using a stretch stitch or the seam will just break when you try to pull it over your belly.
4) Be sure that whatever garment you will be fitting with the belly panel is prepared: lower (under belly) in the front and with finished raw edges. My garment had a lining so I basted the two layers together and finished with a zig zag stitch since I don't have a serger. This fabric doesn't have to be stretchy.
5) Obviously the stretchy panel is a good deal smaller than the non-stretchy garment, so we will need to quarter pin. With right sides together, find center back and match that to the center back of the panel, pin. Next find center front of garment and center front of panel and match those, pin. Do the same on each side. Now, you should be able to see that the slack in your non-stretchy garment is pretty evenly distributed, if not, fix it. You can either continue to pin, using the same method of 'matching centers' or go ahead to the sewing machine.
6) As you sew these two pieces together, you will need to stretch the poly interlock to the width of your garment. The first picture below shows the poly 'unstretched' and the second picture shows the way you should stretch it as you sew. USE A STRETCH STITCH and match the raw edges.
7) Last step! Fold the raw edges toward the belly panel, and topstitch (still using a stretch stitch) and stretching the poly as needed.
Sorry this photo is weird, I was trying to take a picture of myself and it was not particularly successful... I am really looking pregnant... Still, you can see that the fit is nice and smooth and it's really comfortable. I think the neude will be invisible under shirts. I'll add a photo of that when I can get somebody to take one!