The second day of menstruation belongs to the menstrual period. A woman’s menstrual cycle varies from 21-35 days, usually around 28 days, and the period can be divided into 4 phases. In the case of a 28-day menstrual cycle, for example, the menstrual cycle starts from the first day of menstruation and the menstrual period is usually 3-7 days long. The follicular phase starts from day 5 to day 13 of the menstrual cycle. During the follicular phase, the secretion of estrogen increases, causing the endometrium to thicken and the follicles to enlarge under the influence of hormones. The follicles begin to enlarge under the influence of hormones. Around day 14 is the ovulation period, when the eggs are expelled. Around day 15-28 is the luteal phase. The primary function of the corpus luteum is to release large amounts of progesterone. Under the influence of progesterone, the endometrium begins to prepare the embryo for implantation. If embryo implantation does not occur within two weeks, the corpus luteum begins to shrink and disappear, and the concentration of progesterone in the body decreases, leading to the shedding of the endometrium, which in turn causes the next menstrual flow. It is recommended that women pay attention to hygiene during menstruation and change sanitary napkins and underwear regularly to prevent the growth of bacteria and retrograde infection.