Major Events of Menstrual Cycle

Day 1–5: Duration

The cycle officially begins on Day 1 with the onset of menstruation. This phase results from the failure of fertilization in the previous cycle, leading to a sharp drop in hormone levels and the subsequent shedding of the uterine lining.

Step-by-Step Events:

1. The corpus luteum from the previous cycle degenerates.
2. Progesterone and estrogen levels fall sharply.
3. Prostaglandins are released, causing uterine contractions and endometrial breakdown.
4. Blood, mucus, tissue fluid, the degenerated endometrium, and the unfertilized oocyte are expelled through the vagina.
5. Blood loss is approximately 45–100 mL; the endometrial lining thins to about 1 mm.
6. Bleeding stops by Day 5 because fibrinolysin prevents clotting of menstrual blood.

Day 5–13: Duration

During this phase, the body prepares for ovulation by developing a mature follicle in the ovary while simultaneously rebuilding the uterine lining. It is the most variable phase in duration.

Day 14: Duration

This is the most critical phase of the cycle for reproduction. It is essentially a single-day event - the release of the egg from the ovary - triggered by the LH surge.

Step-by-Step Events:

1. Estrogen levels reach a threshold → trigger a positive feedback mechanism.
2. Both LH and FSH reach their peak levels around Day 14.
3. The sudden, rapid surge in LH is termed the LH surge.
4. The LH surge causes the Graafian follicle to rupture.
5. The secondary oocyte(not a mature ovum) is released into the pelvic cavity → this is ovulation.
6. The egg travels into the fallopian tube via the fimbriae.
7. The egg is viable (can be fertilised) for approximately 24 hours after release.

Day 15–28: Duration

After ovulation, the body enters a "waiting phase" to determine whether fertilisation has occurred. The ruptured follicle transforms into a hormone-secreting structure called the corpus luteum, which maintains the uterine lining.