Wrinkles in trousers, at the back as at the front, can be read like a map: every fold points towards the area that is short of fabric or has too much of it. Soft horizontal folds under the bum signal a back crotch that is too long; star-shaped drag lines radiating at the front, from the crotch, give away a crotch that is too short; a waist gaping at the back comes from a deeper sway in the lower back than the pattern allows for; fabric pulling across the thighs simply needs more width. The good news: each of these symptoms matches a precise adjustment, almost always between 0.5 and 3 cm, to make on the pattern rather than to rescue on the finished garment. This guide teaches you to make the right diagnosis in front of the mirror, then to correct it, and shows you why, on a pattern drafted directly to your measurements, most of these adjustments simply no longer need doing.
In short: the crotch seam is the curved seam that runs from the centre front waist to the centre back waist, through the crotch. Its length and shape largely decide how a pair of trousers hangs. When trousers wrinkle, it is almost always the crotch, not your body, that is to blame.
Why do my trousers have wrinkles at the back, under the bum?
These soft horizontal folds, piling up just below the crease of the buttocks, are the most common fault, the classic baggy seat. The diagnosis: the back crotch is too long for you. The pattern allows for more depth through the seat than your figure needs, and the extra fabric drops into an accordion.
The adjustment is made on the back pattern piece. Draw a horizontal line halfway between the waist and the crotch, fold the pattern along this line to take out 1 to 2 cm (start with 1 cm, it is often enough), then redraw the crotch curve and the side seam to smooth out the break. On trousers that are already sewn, the equivalent is to lower the back waistband, but there is little room to play with, which is why it pays to correct the paper.
Ready to go from reading to sewing?
Create a pattern to my measurementsWhat do star-shaped drag lines at the front crotch mean?
Taut folds radiating from the crotch towards the hips, like spokes or whiskers, mean the opposite: the crotch is too short, the fabric is under tension and pulls towards the point under most strain. You see them at the front over a tummy, at the back over a rounded bottom; they get worse as soon as you walk or sit down.
There are two possible corrections, often combined:
- Lengthen the crotch point: extend the crotch point (front or back, depending on where it pulls) by 0.5 to 1.5 cm outwards, then blend the new line into the inside leg seam over about ten centimetres. It is the most effective adjustment for every centimetre invested.
- Increase the crotch depth: cut the pattern horizontally between waist and crotch and spread it 1 to 2 cm apart over a strip of paper, if the whole depth through the hips is lacking.
Work in small steps (on a crotch seam, 1 cm really does change the way it hangs) and test each correction on a toile before you cut your final fabric.
Why does the waist of my trousers gape at the back?
You fasten the trousers, they fit at the hips, but a gap of several centimetres opens between the waistband and the small of your back: it is the sign of a sway back deeper than the pattern's. Your lower back curves in more than average, and the centre back of the trousers stays too high and too straight.
The correction is called a sway back adjustment: on the back pattern piece, lower the centre back at the waist by 1 to 3 cm (measure the real gap by sliding your hand into the back of the fastened trousers), then redraw the waistline, blending it out towards the side, without changing the length of the side seam. If the gaping persists, deepen the back darts slightly, by 0.5 cm each. This family of adjustments is covered in detail in our guide to adapting a pattern to your body shape.
Why are my trousers pulling across the thighs?
Taut horizontal folds across the top of the thighs, fabric that strains and rides up as you walk: there is simply not enough width at the thigh. It is neither the crotch nor the waist; it is a thigh measurement larger than the pattern size allows for, an extremely common situation.
Add 0.5 to 1 cm to each seam (side seam and inside leg, front and back), up to 4 cm in total around the thigh, blending the addition over 15 to 20 cm towards the knee to keep the line. If the difference is more than 4 cm, it is better to redraw the leg from your actual thigh measurement: it is a sign that the pattern size is not the right starting point. Hence the importance of starting from recent, carefully taken measurements, thigh included.
Which symptom matches which adjustment?
The table below sums up the diagnosis. Always read the folds in this order: first their direction (soft and horizontal = too much fabric; taut and radiating = too little), then their position.
| What you see | Likely cause | Adjustment on the pattern | Starting amount |
|---|---|---|---|
| Soft horizontal folds under the bum | Back crotch too long | Fold the back horizontally to shorten | 1 to 2 cm |
| Star-shaped folds at the front crotch | Front crotch too short | Lengthen the front crotch point | 0.5 to 1.5 cm |
| Taut folds under the seat, fabric that digs in when sitting | Back crotch too short | Lengthen the back crotch point | 1 to 1.5 cm |
| Waist gaping at the centre back | Pronounced sway back | Lower the centre back at the waist | 1 to 3 cm |
| Fabric pulling across the thighs | Not enough width at the thigh | Widen the side seam and inside leg | 0.5 to 1 cm per seam |
| Leg twisting round the thigh | Grainline not followed when cutting | Recut, checking the grainline | — |
One golden rule: one adjustment at a time, with a fitting in between. Two adjustments made together mask each other, and you can no longer tell which one did the work.
Adjust, or start from a pattern already drafted to your measurements?
Let's be honest: these adjustments work, but they all correct the same starting problem, a pattern drafted for an average body that is not yours. A long crotch, a sway back, full thighs: none of these "faults" is a fault. They are measurements.
That is where a pattern drafted to your body measurements begins: it works from your waist and hip measurements (and your thigh and inside leg, if you give them), with the crotch depth estimated from your proportions, then confirmed on the toile. Our made-to-measure trousers pattern starts from your figures, not from a size chart: the toile is there to check and refine, not to rescue a size that was never yours.
FAQ: fixing trousers that wrinkle
How can I tell if the crotch is too long or too short?
Soft, sagging folds, visible when you stand still, signal too much length (crotch too long); taut folds radiating from the crotch that get worse as you move signal too little (crotch too short).
Can you adjust the crotch of trousers that are already sewn?
Partly. You can scoop out a crotch that is too short by 0.5 to 1 cm within the existing seam allowance, or shorten a long crotch by taking up the back waistband. Beyond that, there is no fabric to work with: the real correction is made on the pattern, before cutting.
Which adjustment amount should I try first?
The smallest one in the table: 1 cm on a crotch seam, 0.5 cm per seam at the thigh. On these curved areas every centimetre has a visible effect; it is easier to deepen an adjustment than to undo it.
Will the folds disappear when I press them?
No. A good press removes a crease from storage, never a fitting fold: that comes back as soon as the trousers are worn, because it comes from the construction, not the fabric. If the fold returns after pressing, what you need is an adjustment.
My trousers are fine standing up but dig in when I sit down. Is that normal?
It is the sign of a back crotch that is a little short, or of too little ease through the hips: when you sit, your hips spread and need a few more centimetres round than when you stand. Lengthen the back crotch point, then test it sitting down in the toile.
What if your next pair of trousers fitted from the very first try-on? Rather than correcting a standard pattern fold after fold, start from a draft built on your waist and hip measurements, with the crotch depth estimated from your proportions and confirmed on the toile. Draft trousers to my measurements.