The darkening of the vulva and thighs of a girl marks the beginning of sexual maturity. It is only after menstruation that the ovaries start to secrete estrogen and progesterone periodically, and under the effect of estrogen, the skin color of these areas will become darker. This is a normal physiological phenomenon. As long as you keep your vulva locally clean in daily life, wash it once a day and change your underwear diligently, it is also recommended to change your sanitary napkin diligently when you have your period, so as to reduce the chances of reproductive tract infection. In addition, after pregnancy, the vulva, the root of the thighs, and the nipples and areolas of the breasts will be colored.