In clinical work, we often encounter such things. A patient with normal menstrual cycle is asked to come for follicle testing on the 12th day of her period, but she finds that the follicle is expelled; she comes for hysterosalpingography on the 12th day of her period, but she finds that the follicle is already very large and the endometrium is very thick, so she cannot do the imaging. Why did such a thing happen? After careful questioning, we found that the patients did not know the algorithm of menstrual cycle, they thought that menstruation was counted from the first day of cleanliness, so the above mistake would happen. Ma|, Infertility Treatment Center, Qilu Hospital, Shandong University We clinically start the menstrual cycle from the first day of menstruation, that is to say, we count from the first day of blood, the first day of blood is the first day of menstruation; the second day of blood is the second day of menstruation …… Do you understand this time? I hope that when my patients come back, everyone will be able to count their menstrual cycle.