Do I have a sway back? Maybe, maybe not. Other fitting issues can become confused with a true sway back. Perhaps this blog will help you to identify what is happening for you.
1. Prominent buttocks or dominant high hips - If your blouse circumference below your waist is not sufficient to circumvent your high hips or buttocks, the fabric will ride up, pulling from where it can, mainly from the larger, lower circumference. This pushes fabric up at the back waist. The garment needs to be wider at the hip level thereby letting the fabric hang on your hips without pushing up. By attaching your SFD skirt pattern (knowing it fits your waist, high hip and low hip circumference) to your bodice waist edge, you know that the blouse, dress or jacket will hang smoothly around your waist and hips.
2. A prominent bust - If you don't have a large/wide enough dart shape or you've not measured enough length from mid-shoulder over your apex to your waist level, the fabric will pull up at the front waist and cause telltale drag lines toward the back waist. Make sure you've drawn the correct dart shape. Go up a dart size/width if needed. And/or re-measure shoulder over your apex to your identified waist level.
3. Looseness across the back waistline - This can be due to being narrow across your back from side seam to side seam at your waist level. Make sure you haven't measured your waist too loosely. The back waist-fitting dart that aims toward your shoulder blades can be shaped in a curve to help shape and sculpt the bodice, the dart tip can be lengthened toward the shoulder blades and you can consider stitching the side seams with a slight curve.
4. A short or generally high waist level - Make sure to evaluate and eliminate the above fit issues before assuming a sway back issue. Note the illustration to the left showing a normal back compared to a sway back. Posture can also contribute to the sway of your back.
If you do have a sway back you may need to shorten the center back of the bodice but the one alternative is to add a CB seam and actually shape the seam to your swayed back. Yes...stitch the seam in a slight curve that will shape the fabric to your body. You will likely be shortening CB as well, but the curved seam is by far a major par to the solution.
Make sure that at CB you maintain a right angle (90 degree) from CB out about 3/4" - 1" (1.7 - 2.5cm) and then begin your taper.
You will likely have to re-draw the back waist fitting dart to maintain its accurate waist width. The side seam would remain unchanged.
Glenda...the Good Stitch!