I know the fashion industry is really trying hard to come up with great-fitting and stylish maternity pants- especially jeans. Well, back in 2006 when I was pregnant with my daughter, I bought the latest and greatest...a "no-waist" jean from Old Navy. These jeans consisted of an elastic waist in the back and a drawstring for the front. This resulted in pants that fell down in the back and a string that cut into my enormous belly in the front. Other than all that, they are cute jeans.
So I noticed that now, in 2009, the new trend in "no-waist" maternity pants is a normal front with an adjustable elastic waist in the back. You know the kind that is in our toddlers' clothes? Well this was a brilliant idea, because it works! And it is comfortable. So this afternoon, I converted my 2006 maternity jeans into 2009 maternity jeans. Take a look.
Jeans before:

You will need:
Old style no-waist maternity pants
3/4" to 1" wide
braided elastic or
buttonhole elastic 2 buttons, 1/2" to 3/4" wide
seam ripper, scissors, pins, safety pin, sewing machine that does buttonholes, needle and thread
Step 1: Use a seam ripper to open elastic casing. Open on both ends of casing, cut existing elastic at one end and pull out the other end. Cut off this useless elastic.


Step 2: Cut a new piece of elastic. I used 1 inch wide braided elastic. I cut this one a couple inches longer than the back of the jeans.
Turns out, that is more elastic than you will need. Next time I will cut my elastic the length of the back casing because it is going to be pulled/scrunched tighter when worn. 
Ideally, you can use the elastic with the buttonholes in it- the kind found in toddler pants. However, I didn't find it so I made my own.
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Here is my sidebar on making adjustable waist, or buttonhole elastic:
1. Fold over ends of elastic so the cut end is not exposed- about 1/4" then again 1/4". Zigzag stitch a couple times.

2. **You should FIRST practice sewing the buttonholes on a scrap of elastic. Adjust the length and width as needed. Every sewing machine is different, so you are going to have to know how to use your machine for this part. I used a stretch buttonhole stitch. I also sewed each buttonhole twice. Sew buttonholes centered on elastic and in this direction (along length of elastic).

Measure the length of the sewn buttonhole on your scrap. Use that measurement to mark your elastic. Mark elastic for buttonholes with a fabric pen/pencil/chalk/etc. Start about 3/4" from the fold so that the fold doesn't get caught under the buttonhole foot. Leave 1/2" between each buttonhole.

3. As seen above, sew in about 3 to 4 button holes on each end of the elastic. Place a pin on one end of the sewn buttonhole. Seam rip from the other end of the buttonhole up to the pin. The pin ensures you don't rip right through what you just sewed!
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Step 3: Sew buttons to inside of jeans waistband. One on each end of the casing. I can't tell you how to sew on a button. I despise hand-sewing. Just do the best you can! Google it if you really have to.
Step 4: Button one end of your buttonhole elastic to one button. Feed the other end of the elastic through the casing. Best to attach a safety pin to the feeding end and scrunch along until it gets to the other end. Pull out a good amount of elastic and button it to the other button. Try on your jeans and adjust as needed.