Two causes account for nearly all of it: the wrong stitch for a structural seam, and an unsecured chain at the ends.
Check which stitch you used. A three-thread overlock finishes an edge. It is not a construction seam on its own, and a garment made with three-thread seams and no other stitching will open under load. A four-thread overlock includes a second needle line that acts as a safety stitch, and that is what makes it a seam. On knit garments, use four thread for construction.
Check the seam ends. An overlocker cannot reverse, so a chain trimmed flush at each end unravels — usually after a few washes rather than immediately, which is what makes it puzzling. Thread the chain back through the stitching, knot it, or make sure another seam crosses it.
Then check the tension. Looper threads that do not meet at the fabric edge leave the edge uncovered and the stitch pulls open. Needle tension too loose gives long slack loops underneath that let the seam slide.
Then check the seam allowance width. Overlocking too close to the edge leaves too little fabric in the stitch, and on a loose weave the fabric simply pulls out of it. Let the blade trim a consistent amount; do not trim to nothing.
Then the fabric. Loosely woven and fraying fabric holds an overlocked seam poorly. Sew the seam on a sewing machine and overlock the allowance separately, or use a French seam.
On stress points — crotch seams, armholes — reinforce with a straight stitch anyway.
Questions people actually ask
Can I use three thread for seams at all?
On light, low-stress work and where another seam crosses it, sometimes. For any garment seam under load, four thread is the right choice.
My seam opened at the very end only.
That is an unsecured chain. Pull the chain back through the stitching for three or four centimetres with a large-eye needle and trim it.
Last reviewed 2026-08-11