Generally, women have armpit hair before menstruation. Both armpit hair and menstruation in women are affected by hormone levels in the body. When a woman starts to grow and develop, her body will gradually secrete sex hormones as her ovaries grow. The sex hormones include androgens, estrogens and progesterone, of which androgens mainly support the growth of body hair, and estrogens and progesterone promote the onset of menstruation in women. When androgens are secreted in a woman’s body, they stimulate the growth of armpit hair and hair on other parts of the body. However, as the estrogen and progesterone in a woman’s body continue to increase and reach a certain level, only then will the onset of menstruation be triggered. Therefore, women usually have armpit hair before menstruation. If a woman experiences early or late menstruation, it is recommended to go to the hospital for examination and treatment.