So I imagine that there are plenty of maternity pant tutorials out there and also a number of patterns. I've seen a few where you add a band to the top of your pants and others where you can replace your pockets with elastic for a lower rise style. I was given a couple pairs of maternity pants and tried these styles and found that my belly starts protruding below the bands on these pants and so when I decided to convert some of my pants to maternity pants, I did it a little differently. I'm sure this also exists on the internet, but here's how I went about it.
I chose jeans that had a fair amount of stretch to them for comfort. I did this to some jeans I already owned that I didn't feel were worth saving as I've already worn them in quite a bit and also bought a couple pairs of stretch jeans from the thrift store.
I tried on each pair and marked the bottom of where my belly started to protrude and meant that I could not bring the fly further up with some tailor's chalk. I then drew a curved line connected this point to the top of the waist band above the side seams.
I cut along this line, carefully cutting through the zipper.
I removed the zipper left on the jeans using scissors and a seam ripper. If you try and sew through the zipper you break your needles on your machine. Ask me how I know this.
This is a different pair of jeans. You can see here that I pinned the pockets to the front leg piece to keep everything in the right spot while I was working. Also note that it's wise to cut so you don't end up having any rivets in your seam allowance. This will also break your sewing machine needle. Ask me how I know this.
To make a pattern piece for the front of the belly band, I took some tracing paper and traced half of the shape of the cut out onto it. I used the centre back seam to draw the mid line.
To add the portion that goes over the belly, I extended the mid line 5 inches. 5 inches is just personal preference for how high I wanted my band to be, you can do whatever length you find most comfortable. I extended the side of the pattern to the same height but tapered in by about half an inch at the top to keep the pants snug.
To make the pattern piece for the back of the belly band, I traced the front pattern piece excluding the front cut out.
I added marking to cut along the fold and to cut 2 of each to each pattern piece.
I cut 2 of each front and back on the fold. I used a stretch technical fabric that was quite soft. It feels like LuLu Lemon leggings fabric. You want something with a fair amount of stretch (>40%), that feels good against sensitive belly skin, has good recovery, and breathes (no one wants a sweaty stomach). The stretch should run horizontally along the waistband.
Pin (using ballpoint needles) the 2 front pieces together with right sides facing. Serge (or you could zig zag) along the top edge.
Pin the 2 back pieces together with right sides facing. Serge (or you could zig zag) along the top edge.
Pin the front and back pieces together with right sides facing and serge along the straight edges on each side making a tube.
Turn right side out. You now have a tube that looks like this in the front when folded in half at the waist seam.
Pin belly band to to the jeans along the belly cut out and along the waist at the back of the jeans. Make sure to remove the pins that you used to keep the pockets in place so you don't sew over them.
It looks weird and ungainly but the stretchy fabric makes it easier. Sew along the pinned edge using a serger or zig zag stitch. I used a serger but it was a bit bulky and messy at times, you might have better control with a zig zag.
All done! It looks super weird without a shirt but you could also use matching fabric to minimize that fact. I did about 5 pairs of jeans assembly line style and it only took a few short sessions. I realized after the fact that I now have 1 pair of black jeans and 4 pairs of jeans in various shades of red/pink/purple but we'll call it a planned wardrobe decision.
Have you ever made your own maternity pants? What worked best for you?