NCERT · Class 12 · Biology · Human ReproductionExplain the menstrual cycle in human females in detail. Discuss the various phases of the cycle along with the cyclical changes occurring in the ovaries and uterus, and the hormonal regulation involved.

Introduction to Menstrual Cycle\nThe reproductive cycle in female primates (e.g., humans, apes, and monkeys) is called the menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. One ovum is released (ovulation) in the middle of each menstrual cycle.

Phases of the Menstrual Cycle\nThe menstrual cycle comprises the following four distinct phases:

1. Menstrual Phase (Days 1-5):
   • If the released ovum is not fertilized, the corpus luteum degenerates.
   • This causes a sharp decline in the levels of progesterone and estrogen.
   • The lack of these hormones leads to the breakdown of the endometrial lining of the uterus and its blood vessels, which leads to bleeding (menstruation), lasting for 3-5 days.
2. Follicular Phase / Proliferative Phase (Days 6-14):
   • During this phase, the primary follicles in the ovary grow and mature into Graafian follicles.
   • Concurrently, the endometrium regenerates through proliferation under the increasing secretion of estrogens by the growing ovarian follicles.
3. Ovulatory Phase (Around Day 14):
   • Both LH and FSH attain a peak level in the middle of the cycle (about the 14th day).
   • Rapid secretion of LH induces the rupture of the mature Graafian follicle and thereby the release of the ovum (secondary oocyte), a process known as ovulation.
4. Luteal Phase / Secretory Phase (Days 15-28):
   • Remaining parts of the Graafian follicle transform into the corpus luteum.
   • The corpus luteum secretes large amounts of progesterone, which is essential for the maintenance of the endometrium.
   • If fertilization does not occur, the corpus luteum degenerates into corpus albicans, initiating a new cycle.

Hormonal Regulation

• Gonadotropins (LH and FSH): Fluctuate during the cycle, peaking at mid-cycle (Day 14) to trigger ovulation.
• Ovarian Hormones (Estrogen and Progesterone): Estrogen dominates the follicular phase, while progesterone dominates the luteal phase to prepare the uterus for implantation.