What is the follicular phase? The follicular phase refers to the most important part of a woman’s menstrual cycle, when follicles grow and develop in the ovaries. This period usually lasts 12 to 14 days. During this period, the follicles continue to grow, and the largest of them will rupture during ovulation, releasing the egg. If the egg is fertilized by sperm during this period, a fertilized egg will form and the woman will have the possibility of pregnancy. How do women determine their follicular phase? Women can calculate their follicular phase by observing their menstrual cycle