My periods are always early. What's going on?

Normal menstruation, lasting 2~7 days, the general menstrual cycle is about 28 days, less than 21 days for menstrual frequency, more than 35 days for menstrual sparsely. The difference between different menstrual cycles is less than seven days. If menstruation is always ahead, but the menstrual cycle is within the range of 21~28 days, it is not a problem, but there may be insufficient luteal function. Luteal insufficiency refers to follicular development and ovulation in the menstrual cycle, but insufficient progesterone secretion during luteal phase or premature luteal decline, resulting in poor endometrial secretion and shortened luteal phase. Generally manifested as shortened menstrual cycle, easy to cause infertility, or abortion in early pregnancy.