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How are follicular and luteal phases different in the menstrual cycle?

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Detailed cross-sectional illustration of a human ovary showing the stages of follicle development, ovulation, and corpus luteum formation, with labeled structures including primordial, primary, secondary, and mature follicles, a ruptured follicle releasing an ovum, and corpus luteum and albicans. An inset at the bottom shows the ovary's location in the female reproductive system. — ovary The steps of ovulation, beginning with a dormant primordial follicle that grows and matures and is eventually released from the ovary into the fallopian tube.

The menstrual cycle is characterized by two primary phases, the follicular and luteal phases, which are separated by ovulation.1

The Follicular Phase

Lasting approximately 14 days, this phase begins with the first day of menstruation. Under the influence of follicle-stimulating hormone (FSH), several ovarian follicles mature. These follicles secrete estrogens, which cause the endometrium (uterine lining) to thicken and proliferate. This phase concludes when a surge in luteinizing hormone (LH) triggers the rupture of a mature follicle and the release of an egg.213

The Luteal Phase

Following ovulation, the ruptured follicle transforms into the corpus luteum. This structure secretes large amounts of progesterone and some estrogen, inducing the secretory phase in the endometrium to prepare for potential implantation of a fertilized egg. If fertilization does not occur, the corpus luteum degenerates, causing hormone levels to drop. This withdrawal leads to the shedding of the endometrial lining, marking the start of a new menstrual cycle.2341