Preshrunk garments are items of clothing that have already been washed, or “compacted,” so additional washings will not shrink the garment significantly. In many cases, when you have natural fibers like wool, cotton, or linen, the fabric may be stretched, since it is more economical to use less fabric if you can get away with it. Washing these garments, and the tumbling action in the dryer will cause the garments to go back to their unstretched form, resulting in smaller clothes.
Since it’s helpful to buy a clothing size that will still fit after washing, you’ll note that many fabrics, particularly cotton jersey, are preshrunk. This means the fabric, instead of being stretched, has first been washed, and pushed together in a process called compaction. The fibers are forced closer together, and gradually become looser through home washing. This doesn’t mean a preshrunk garment won’t shrink at all, but it will shrink less, especially with early washings, and will maintain the same size for a much longer time than a garment that is not preshrunk.
Eventually, especially when clothes are dried in the dryer, even preshrunk clothes will shrink somewhat, given enough washing. To address this, it’s a good idea to buy cotton garments in a slightly larger size than you would normally need, especially cotton jersey garments. Cotton pants, especially corduroy are generally not preshrunk, and will gradually get shorter as they are washed. Another notorious shrinking fabric is cotton flannel. Always buy flannel nightgowns a size or two larger than what you think you need.
Manmade fibers have frequently eliminated the need for garments to be preshrunk. Adding enough polyester to cotton, for instance, will usually help a garment retain its shape and size, since polyester holds its form better in the wash. The disadvantage of polyester, nylon and other manmade fibers is that they may not be as comfortable, and are generally less breathable than are natural fibers.
If you’re really crazy about cotton, wool or linen, and want to stay away from fibers like acrylic or polyester, look for labels stating that the clothing is washable or that boast the clothing has been preshrunk. All clothing tends to wear better, last longer and maintain its size if you do not place it in the dryer. So if you have non-preshrunk garments, consider drip or line drying only. Note that even preshrunk garments tend to shrink eventually, so you may want to observe this caution with any garment for which you want to maintain the same shape, length or size.