Underarm melanin deposition may be a normal phenomenon due to pregnancy, and may also be caused by acanthosis nigricans, with different causes and different treatment methods. 1. Physiological factors: hormonal changes in pregnant women’s bodies during pregnancy can lead to hyperpigmentation in the areas of long pubic hair and armpit hair, which can lead to underarm melanin deposition, usually without special treatment, and it can get better by itself after the end of pregnancy. 2. Acanthosis nigricans: Acanthosis nigricans is a kind of excessive skin pigmentation, velvety thickening of the disease, the disease can also lead to the manifestation of underarm melanin deposition, the treatment can be prescribed oral vitamin A, topical urea ointment, retinol ointment, etc. for treatment. Excluding physiological factors, if the patient’s axillary melanin deposition symptoms continue to be unrelieved, should be timely consultation, identify the cause, please doctor to be disposed of.