The follicular phase generally refers to the stage of follicular growth, including the basal follicle, pre-sinus follicle, sinus follicle, and pre-ovulatory follicle, which usually takes about 15 days and corresponds to the proliferative phase of endometrial changes. 1. Primordial follicle: The follicle in this period is usually a primary oocyte that stays in the bilinear phase of meiosis and is surrounded by a single layer of spindle-shaped pre-granulosa cells on the outer layer of the cell. 2. Pre-sinus follicle: At this time, the primary follicle is formed, the zona pellucida appears around the egg, and follicle stimulating hormone receptors, estrogen receptors, and androgen receptors appear in the granulosa cells, and luteinizing hormone receptors appear in the inner follicular cells. 3. Sinus follicle: due to the combined effect of estrogen and follicle stimulating hormone, follicular fluid increases, follicular cavity is formed, and the pre-sinus follicle develops into a sinus follicle. 4. Pre-ovulatory follicle: follicular fluid continues to increase in this period, the follicular cavity gradually increases, the volume of the follicle grows, and structures such as the follicular epithelium, endofollicular membrane, and radial bands appear.