What is a follicle? A follicle is located within the ovarian cortex (that is, on the surface of the ovary) and a follicle is composed of an oocyte and its surrounding follicular granulosa cells. Follicle development and maturation The number of follicles varies during different periods of female growth as follows: 7 million during fetal life, 2 million at birth, 300,000-500,000 during puberty, and basically depleted after menopause. Only 400-500 follicles are generally able to develop and ovulate during a woman’s lifetime. Only one or two dominant follicles in this group can fully develop and ovulate, while the rest of the follicles degenerate and disappear during the development process. According to the morphology, size, growth rate and histological characteristics of the follicles, the follicles can be divided into four stages: the primordial follicle, the antral follicle, the sinus follicle and the preovulatory follicle. From the first day of menstruation to the maturation of the follicle, called the follicular phase, which usually takes 10-14 days. Ovulation The follicles become mature when they reach 18 mm or more. At this time, the oocytes inside the follicles are squeezed and expelled, which is called ovulation, which occurs about 14 days before the next menstrual period. Luteal formation and degeneration After the oocyte is expelled, the fluid inside the follicle also flows out to the lowest point of the human pelvis (Douglas’ fossa), i.e. the rectal sink of the uterus. Therefore, when ultrasound is used to monitor whether the follicle is expelled, if fluid is seen in the pelvis, it is considered that the follicle has ruptured and the oocyte is expelled. After the oocyte is expelled, the remaining tissue forms the corpus luteum. Progesterone peaks on the 7th-8th day after ovulation, around the 22nd day of menstruation, at which point it can be monitored in the blood for elevated progesterone to determine if ovulation has occurred. After ovulation, if the egg does not unite with sperm in time to form a fertilized egg (the future child), the corpus luteum begins to degenerate 9-1O days after ovulation. The luteal phase is from the day of ovulation to the onset of menstruation, usually 14 days. After the luteal function declines, menstruation begins, when new follicles develop in the ovaries and a new cycle begins.