Small holes in a line just under a waistband are a common problem on fine knit underwear, and modal jersey is more sensitive than heavier cotton. The position of the holes is the first clue. If they follow the stitch line, the cause is usually at the seam: needle damage during sewing, or stress concentrated where the stiff waistband meets the softer body fabric. If holes appear randomly across the body, fabric strength or wear is more likely.

Look for needle damage

A needle that is too thick, blunt or the wrong point type can cut or weaken the knit loops as it sews. The damage may not show as a hole straight away, but opens up after stretching and washing. Ask the factory which needle size and point it uses on the waistband operation, and how often needles are changed. Ballpoint needles are commonly used on knits to push loops aside rather than pierce them. Inspect new, unwashed garments under good light while gently stretching the seam area; early damage shows as broken loops beside the stitches.

Check seam and elastic tension

If the waistband elastic is much stronger than the body fabric, every time the waistband is pulled the force concentrates on the fabric just below it. Compare the elastic's force at a set extension, using a method such as ASTM D4964, with what the body fabric can take. A very tight stitch or a stitch with too little stretch can also cut into the fabric when the seam is extended.

Test the fabric itself

Measure the body fabric's bursting strength with a named method such as ASTM D3786, on new fabric and after several washes under a named procedure such as ISO 6330. Agree a minimum with the mill. Modal blends can lose strength when wet, so do not judge the fabric from dry handling alone.

Decide and fix

If the damage starts at the needle, change the needle size or type and set a needle change routine. If it starts from stress, consider a softer elastic, a wider waistband join or a different stitch. Approve a washed and stretched sample before the next bulk run, and keep it as the reference.