There are three ways you can go about sewing a maternity pant:
1. Adapt an existing non-maternity pant that you can pull over your hips,
2. Adapt a sewing pattern for a non-maternity pant , and
3. Draft a pant pattern yourself and adapt it to use while pregnant.
Either way, the steps you go through are about the same.
I will begin with some notes on the fabric for the project:
1. Pant fabric: Regardless whether you are sewing the pant or using a RTW one, you want the fabric to have at least 5% crosswise stretch to it. I used the word "want," because you don't have to go with stretchy fabric. Nonetheless, crosswise stretch will be practical and more comfortable.
2. Fabric for panel: must have 4-way stretch and must be nice on touch.
1. Pant fabric: Regardless whether you are sewing the pant or using a RTW one, you want the fabric to have at least 5% crosswise stretch to it. I used the word "want," because you don't have to go with stretchy fabric. Nonetheless, crosswise stretch will be practical and more comfortable.
2. Fabric for panel: must have 4-way stretch and must be nice on touch.
Starting point:
1. If you are not going to be drafting your pattern, choose a pattern for a non-maternity pant. You must choose one according to your current hip measurement (the waist measurement really doesn’t matter). Also, needless to say, a patern with a lot os details at the waist or just below, will be impractical. The pattern must be for a flat-front pant. The back pockets can be preserved, however, the front pockets will have to be eliminated.
2. If you are using a RTW pant, it must fit you around the hips (the fit at the waist is irrelevant). Additionally, it must be a flat-front pant, and idealy will have no front pockets.
Next, you must prepare the pattern/RTW pant:
1. If you are sewing the pant (self-drafted or not), make the muslin leaving the top 3” from the side seams unsewn. Also, sew only about 3.5” from the front crotch. At this point you have to do a fitting and determine how the pants fit you in the hips and thighs, and do the necessary alterations .
2. If you are using a RTW pant, remove the waistband, unstitch the top 3” from the side seams, undo the front closure, remove the front zipper, and keep only about 3.5” from the front crotch sewn up.
Next, you have to establish where the "waistline" would be (at this point, a fitting buddy or even a willing husband/bf could be helpful, but I have done it by myself). Bear in mind that this is not a regular waistline, and I am using the word loosely. What I am refering to here and below, is the line where the pant will connect with the maternity panel. To determine the location of this waistline, mark a line that:
1. On the front: is just under the lowest part of your pregnant tummy. It is very easy to tell since the line should be right under the pregnancy bulge,
2. On the back: start from the side seam and continue in an upward direction aiming to hit the point of your natural (pre-pregnancy) waist at the CB.
The line (especially on the front) can look crooked. Straighten it as much as you can without disturbing the natural curvature of your body.
At this point you either transfer the marks on your pattern or remove what is above the marked line on your RTW pant (but remember to leave 0.5" for seam allowance).
Next, you have to make the maternity panel. I prefer the extended version that goes to just below your breasts, but you can go with any height you consider comfortable, even with a fold-down. The instructions below are for the extended version, however, it is easy to infer from them how to make a less high panel or a fold-down one.
You can buy lycra fabric (or other fabric with great recovery) for the panel. I prefer to use a seamless RTW lycra tank top. Alternatively, you can buy a maternity band from Target (a high-quality knock-off of the Bella Band). The advantage to using a tank top or a maternity band is the seamless-ness. If you are making the panel from fabric, decide on the width: it must go around the widest pat of your belly, be snug, but not compressing.
Now, you must put on the pants and pull your self-made panel/tank top/maternity band over your belly. The bottom of the panel must be over the previously marked waistline. Using chalk or a marking pen, mark the waistline from the pant on the panel. Next, determine how high you want your panel to be, and chalk-mark it on the panel. Make sure to add 0.5" seam allowance at the bottom line of the panel and 4" seam allowance at the top. If you are using a maternity band, you can use the top end of it and not change anything about it (it is tight as it is, and does not need elastic to stay up). Of course, if you are using a maternity band, the top line of the panel wil be straight, as opposed to curved (as on the sketch below). I prefer a curved line, for it covers the belly more adequately. Therefore, if you want a curved top line, use a self-made panel ot a tank top.
1. If you are making your own panel: sew the single side seam first,
2. Fold under 2” and zigzagg/topstitch it down,
3. Insert 1.5” wide elastic in the casing. If you want a narrower elastic, adjust the casing accordingly.
Lastly, you have to attach the panel to the pant. I'd use my serger to do so, however, if you don't have a serger, you can do it with your sewing machine using a small zigzag.
Here is a close-up of the extended panel attached:
This is how the pants look under a shirt (the panel is completely concealed):
This technique can be used for a skirt as well.