WebMD indicates that female mustache growth is a common symptom of polycystic ovary syndrome, which is a condition that causes women to create extra male hormones. This leads to excessive hair growth on the face, back, stomach and chest. Additional symptoms include irregular menstrual periods, fertility problems and weight gain. The overall cause of polycystic ovary syndrome is not understood, but genetic factors are often linked to the condition.
Healthline explains that it is normal for women to grow light hair on the face, which is often referred to as "peach fuzz." Excessive facial hair growth is referred to as hirsutism. The hair growth patterns of hirsutism in women are similar to male hair growth, and the hair's texture is coarse. Causes for male hair growth patterns in women include steroid or testosterone injections, adrenal cancer and Cushing's disease. Cushing's disease is a condition in which the body produces excessive levels of cortisol, a male hormone that is often referred to as the "stress hormone."
According to the Mayo Clinic, later symptoms of hirsutism in women include male pattern baldness, a deeper voice, acne and a decrease in breast size. In addition to excessive male hormones, such as testosterone, hirsutism is also linked to genetics and family traits.