I found luvinthemommyhood’s Maternity Sewing Roundup to be invaluable in figuring out how to do this. I read all of the pant and skirt links, but in particular I drew the most from the refashioned maternity skirt and the maternity jeans tutorials.
I tried to convert my pattern by making measurements on some maternity pants, but it ended up getting too confusing. Making a muslin of the skirt pattern was the most straight forward way to figure it out. (Incidentally, this means you can use this method to change an existing skirt to maternity as well.) Try on your muslin and trace around the edge of the belly.
This pattern is an a-line skirt, so where the bottom of my bump starts there is considerable “ease”. I pushed the line up a bit to a part of the skirt that was closer to my body.
Next, I traced it on my pattern pieces, cleaning it up as I went. I had to make a copy of the side front/back piece as the changes where different for the front to back. I forget to add a seam allowance to my waistline. oops! (I cut down my pattern after taking this picture.)
Once the fabric pieces were cut from the pattern, I assembled them according to the original directions (except leaving off the zipper). Then, I tried it on. You need to make sure that you can get your skirt on without a zipper. I intentional chose stretch denim to help with that, but I think it ended up being unnecessary. If you can’t get it on, make the waist opening larger. Lowering the front will add width and so will dropping the waistline farther down if your skirt is a-line. I discovered my waist was way too big and took in a total of 2″ from the seams.
There really should be a picture here, but I was in a sewing groove and forgot to stop and snap one. sorry!
Once you are happy with the waist opening, it is time to make your waistband. The material I used I am really happy with and would recommend it to you. It is a cotton/spandex tubular rib knit. It is sturdier than t-shirt jersey and the spandex helps it recover well. I didn’t even need to add any elastic to the waistline to help it hold up the skirt. I found it at Joann’s.
I cut a piece of the ribbing 16″ x 32″. 16″ gives unrolled height of about 7 1/2″, and 32″ is about 3/4 of the waist opening of my skirt. I arrived at the 3/4 ratio by trial and error. I found it snug enough to hold up the skirt, but with enough stretch left to last me through to the end. When cutting your waistband, be sure to cut it so the waistband width is along the stretchiest direction. This is usually (always?) across the width of the fabric (selvage to selvage).
Fold waist band in half width-wise and sew into a tube using a stretch stitch. Then fold it in half along its height to enclose the seam you just made. (see modest maven’s tutorial, steps 2 – 3, if my lack of pictures is totally confusing.)
Pin along the waist opening of your skirt. The waistband is smaller than the skirt waist, so you will need to distribute that difference evenly along the entire edge.
Sew the waistband to the skirt. I chose a stretch stitch because my skirt fabric has width-wise stretch. Try it on! If you are still happy with it, you can finish up the waistband edges with a overlock or zig-zag stitch.
Hem according to original directions and you are done!