If you’ve ever noticed pale patches or unexpected discoloration on your underwear, you may have wondered whether detergent, fabric softener, or the washing machine was responsible. Surprisingly, the cause is often much more personal—and completely natural. Vaginal discharge is part of the body’s normal process for maintaining reproductive health. The vagina naturally produces discharge to remove dead cells and help maintain a healthy internal environment.

One important part of that environment is acidity. Beneficial bacteria, particularly lactobacilli, produce lactic acid that helps keep vaginal pH within its normal range, generally around 3.8 to 4.5. When this mildly acidic discharge repeatedly comes into contact with fabric, it can interact with certain dyes and gradually cause lighter patches, especially on darker underwear. But there is an important reason this happens that may actually be reassuring.