Darkening of the nipples refers to the aggregation of melanin granules in the melanocytes of the nipple skin, which is in fact a kind of pigmentation. The pigmentation of the human body skin is mainly related to the hormones in the hypothalamic-pituitary-gonadal axis. In women, as they enter puberty, the function of the ovaries starts to activate, and a large amount of estrogen entering the bloodstream will act on the sexual organs, such as the breasts, pubic area, and armpits, causing hyperpigmentation in the corresponding areas. During pregnancy and lactation, this hormone level will be higher and may cause more severe hyperpigmentation, and the degree of hyperpigmentation in these specific areas varies from person to person. Some people may have very pronounced hyperpigmentation, others may have less pronounced hyperpigmentation, and some people may have less hyperpigmentation as their hormones slowly return to normal as they finish breastfeeding.