So this tutorial will use steps from my other pants tutorials, rather than having an epic long post.
The referenced steps will have links if you want to click over to see the pictures.
The photographed steps here are mainly for the real adult front pockets.

So to get started:

FABRIC:

you'll need anywhere from 2-4 yards of fabric depending on how long your inseam is, and how wide your fabric is.
45" wide fabric you'll need more like 3-4 yards twice the length of your finished jeans.

60" wide fabric you can cut side by side and only need the length of your pants most likely.

1. TRACE JEANS YOU LOVE FOR PATTERN

I was making a few pairs, so I traced my favorite fitting jeans on paper so I had a pattern to use multiple times.  But you could also just go right on the wrong side of the fabric.
Outline your perfect fitting jeans exactly on the seams, and you'll have to adjust/ rearrange them as you draw to get the curve of the butt and crotch to lay flat so you can outline it.

**I outline below the jeans waistband because you don't need that as you'll add the elastic or panel at the end**

Most likely the back of your jeans has a yoke, or triangle piece at the top of the butt, don't worry about that and just outline below the waist band.

Don't forget to also trace the back pockets and the curve of the front pockets.

2. ADJUST OUTLINE OF PERFECT JEANS FOR MATERNITY

So once you've got the front and back halves of your jeans drawn, you need to first add seam allowances.

I like to sew with 1/2" seam allowances, but my good fitting jeans were currently too small with the pregnancy weight gain, so I also added another 1/4" to all sides to up the size for the growing mama legs and butt.

So from the original exact jeans outline, I added 3/4" to the sides, top, and curves of butt/ crotch.

For the bottom of the legs, I add 1.5" to the for a hem.

Back pockets only need 1/4" seam allowance on sides, 1" seam allowance on top to fold over.

Now the back pieces are done as far as drafting goes.

But the front needs a maternity make-over.

1. Add the faux fly by going beyond the curve 1.5" and curving back into the center front crotch seam.

2. Trace where the front pockets curve and add 1/4" seam allowance.

3. The red line below indicates where the jeans would be just below the waistband.
You'll need to lower the front of the jeans to allow for the shorter rise for the baby bump.  I just guess, usually making the total front crotch seam 6" for me.

**You'll also need to cut out the pocket backing pieces, but instructions for those pieces are included below in sewing instructions.

3. Sew Back of Jeans

here's where I rely on past tutorials

images of these steps on baby jeggings can be found here under "construct back of pants" heading

1. Back Seam. 

Sew (with right sides together) the curved back bum seam.  Finish raw edges (serge/ zig-zag).  On right side, sew the flat felled type seam by pushing the 1/4" seam allowance to one side and top stitching 1/4" from seam fold to stitch it in place.

3. Attach back pockets.  

First fold pocket top down 1/2" twice to create the final 1" hem.  You could also serge the top and fold it down once 1".  Top stitch the top fold.  Do any embroidery/ applique on pocket now if you want.  Iron the sides of pocket to wrong side with 1/4" seam allowance.  Mark the pocket placement using your original jeans.  Pin pockets on pants and top-stitch around sides and bottoms with 1/8" seam allowance.

4. Sew Front Center Seam and Faux Fly

With right sides together on front pieces, sew the curve of the faux fly and then pivot down the center front (crotch) seam.  You'll snip the seam allowance to the stitching at the corner where you pivot.

Open jeans and fold the faux fly back to the left if you were wearing them, right if you're looking at the front of the jeans.  (Most of the time I just look down at the pants I'm wearing to make sure the fly goes the right direction)

Top stitch the curve of the fly, pivoting at the center front seam and top-stitching that down.

Usually jeans have the double needle top-stitching, or you can sew it twice 1/4" away from the first seam.

5. MAKE FRONT POCKETS

You'll need to cut out the backing and lining for the front pockets.
Rather than using the denim (corduroy) for the whole pocket, you'll notice in adult jeans the bottom half of the pockets are always a lighter fabric so you don't see the bulky outline of the pocket smashed on your thigh.

So first cut the backing of the pocket from your jeans fabric, cutting up the front curve to meet the side seam of the back piece.  Then make the piece long enough to go into pocket.
You'll need two of these pocket backing pieces, opposite each other for a left and right pocket.

Next, you'll need to cut the pocket lining for the bottom back half and front of the pocket.
I used quilting cotton and cut two strips as wide as my pocket backing pieces.
The length of this pocket lining will determine how deep your finished pockets are, as this lining is sewn to the pocket backing and front of pants, then folds between to make the pocket.
I used my original pants to measure the approximate length for my pockets.

So when cutting the lining out, (far left image) you'll have one end that matches the curve of the pocket backing, then the other end will match the curve of the pocket on the front of the pants.
Once you cut out one lining, use it to cut the opposite.

Sew the lining to the pocket backing (center image) with right sides together.  Fold lining down and press, and I top-stitch this seam as well to help it lay flat.

You can see now how the lining will fold up to create the pocket when you attach it to the front of the pants (right image).

6. ATTACH POCKETS TO FRONT OF PANTS

Once lining is sewn to pocket backing, you can sew the other end to the front of the pants.

Make sure you're sewing right sides together and have the correct pocket (right pocket to right side of pants).  

Flip the pocket back and top-stitch the front of the pants along the pocket curve, keeping the lining back so it won't show on the front.

At this point you can align the pocket with the front of the pants, lining the curve of the front and side seams all together.

7. FINISH POCKETS AND PANTS FRONT

At this point you can see where the pockets will align with the front of the pants and pin where the pockets will fold.  Next sew up the side of the pocket near the center front.

For the outer side of the pocket, I sew it right to the front of the pants, making the pocket included in the side seam so it can't twist toward the crotch of the pants if you have something in your pocket.

Baste the top of the pocket to the front of the pants, and the side seam.

 Now all you have to do is clean up the curve along the front of the pants, I chose to serge it in a clean curve.

Inner Leg Seams:

Pin the front to back (right sides together) matching up the center crotch seams.  Pin the legs together at the ankle.  I start from the center and sew to one ankle, then repeat.  If it ends up a little (.25") uneven at the ankle, it's alright.

Open this seam and top-stitch seam allowance to one side for the flat felled finish seam look.

 Side Seams:

With front and back right sides together, sew the side seams and finish seam.  Make sure you catch all the sides of your front patch pockets to enclose the raw edge in the side seam.

Hem:

Go ahead and hem the ankles folding the raw edge twice.  I allowed 1.5" extra when cutting out for a 3/4" double fold hem.

9. ATTACH MATERNITY BAND OF CHOICE

Here's a tutorial at this point if you want to add a knit covered elastic band.

I hate the full panels, so I always go with a band that sits below the belly.

For these orange jeans I just used 4" wide elastic, sewing it right to the jeans, not even taking time to create the knit cover for it.

Obviously I'd never tuck in maternity shirts, but just to show you how I just plopped on thick elastic to the pants:

 So there you go, making your own maternity jeans that should fit like your favorite pre-pregnancy jeans you can't jam your thighs in anymore (if you're like me).

So now I have the plum maternity cords (with only faux pockets) and these orange babies...

...and only 12 weeks to go till she's here.

That seems like a long time, the whole last trimester.