Menstrual Cycle — Revision Notes

⚡ 30-Second Revision

Cycle Length: — Average 28 days (range 21-35).
Phases: — Menstrual (1-5), Proliferative (6-14), Ovulatory (Day 14), Secretory (15-28).
Hormones & Source:

- Hypothalamus: GnRH - Ant. Pituitary: FSH, LH - Ovary: Estrogen, Progesterone

Key Events:

- FSH: Follicle growth. - Estrogen: Endometrial proliferation, LH surge trigger (high levels). - LH Surge: Triggers ovulation. - Ovulation: Egg release (Day 14). - Corpus Luteum: Forms post-ovulation, secretes Progesterone & Estrogen. - Progesterone: Endometrial secretory activity, maintains pregnancy, negative feedback. - Menstruation: Due to ↓ Progesterone/Estrogen.

2-Minute Revision

The menstrual cycle is a monthly preparation for pregnancy, averaging 28 days, starting with menstruation. It involves coordinated changes in the ovaries (ovarian cycle) and uterus (uterine cycle), driven by hormones from the hypothalamus (GnRH), anterior pituitary (FSH, LH), and ovaries (estrogen, progesterone).

Ovarian Cycle:

1
Follicular Phase (Days 1-14): — FSH stimulates follicle growth. Developing follicles produce estrogen, causing endometrial proliferation.
2
Ovulation (Day 14): — High estrogen triggers an LH surge, leading to egg release.
3
Luteal Phase (Days 15-28): — Ruptured follicle forms corpus luteum, secreting progesterone and estrogen. Progesterone prepares the uterus for implantation.

Uterine Cycle:

1
Menstrual Phase (Days 1-5): — Shedding of uterine lining due to drop in progesterone/estrogen.
2
Proliferative Phase (Days 6-14): — Endometrial regrowth under estrogen.
3
Secretory Phase (Days 15-28): — Endometrial maturation (thick, vascular, secretory) under progesterone.

If no pregnancy, corpus luteum degenerates, hormone levels fall, and menstruation begins again. Remember the positive feedback of estrogen for LH surge and the constant 14-day luteal phase.

5-Minute Revision

The menstrual cycle is a complex, hormonally regulated process in females, typically 28 days long, preparing the body for potential pregnancy. It's divided into ovarian and uterine cycles, intricately linked by the hypothalamic-pituitary-ovarian axis.

I. Hormonal Control:

Hypothalamus: — Releases GnRH (Gonadotropin-Releasing Hormone).
Anterior Pituitary: — Stimulated by GnRH to release FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone).
Ovaries: — Produce Estrogen and Progesterone under FSH/LH influence.

II. Ovarian Cycle (Changes in the Ovary):

1
Follicular Phase (Days 1-14): — FSH stimulates the growth of several ovarian follicles. These developing follicles secrete estrogen. Estrogen levels gradually rise, initiating the repair and proliferation of the uterine lining.
2
Ovulation (Around Day 14): — A peak in estrogen levels from the dominant follicle triggers a positive feedback loop, causing a massive LH surge (and a smaller FSH surge) from the anterior pituitary. The LH surge is the direct trigger for the rupture of the Graafian follicle and the release of the secondary oocyte.
3
Luteal Phase (Days 15-28): — After ovulation, the ruptured follicle transforms into the corpus luteum under LH influence. The corpus luteum secretes large amounts of progesterone and some estrogen. Progesterone is crucial for preparing the uterus for implantation.

III. Uterine Cycle (Changes in the Uterus):

1
Menstrual Phase (Days 1-5): — If no fertilization/implantation, the corpus luteum degenerates, causing a sharp drop in progesterone and estrogen. This hormone withdrawal leads to the breakdown and shedding of the functional layer of the endometrium, resulting in menstrual bleeding.
2
Proliferative Phase (Days 6-14): — Overlaps with the follicular phase. Rising estrogen levels from the developing follicles stimulate the regeneration and thickening of the endometrium, preparing it for a potential embryo.
3
Secretory Phase (Days 15-28): — Overlaps with the luteal phase. Progesterone from the corpus luteum causes the endometrium to become highly vascularized, glandular, and secretory, producing nutrient-rich fluid to support an early embryo.

Key Takeaways for NEET:

Positive Feedback: — High estrogen → LH surge → Ovulation.
Negative Feedback: — Moderate estrogen/progesterone → inhibit GnRH, FSH, LH.
Luteal Phase Length: — Relatively constant at ~14 days. This helps calculate ovulation day (Cycle Length - 14).
Corpus Luteum Fate: — Degenerates if no pregnancy, maintained by hCG if pregnant.

Worked Example: If a woman has a 32-day cycle, on what day would ovulation most likely occur?

Solution: — Ovulation occurs approximately 14 days before the next period. So, $32 - 14 = 18$. Ovulation would likely be around Day 18.

Prelims Revision Notes

The menstrual cycle is a monthly series of changes in the female reproductive system, averaging 28 days, starting from the first day of menstruation.

I. Hormonal Regulation:

Hypothalamus: — Releases GnRH (Gonadotropin-Releasing Hormone).
Anterior Pituitary: — Releases FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone).
Ovaries: — Produce Estrogen and Progesterone.

II. Ovarian Cycle:

1
Follicular Phase (Days 1-14):

* FSH stimulates follicular growth. * Developing follicles secrete Estrogen. * Estrogen causes endometrial proliferation.

1
Ovulation (Day 14):

* High Estrogen levels cause positive feedback on pituitary. * Leads to LH surge (and minor FSH surge). * LH surge triggers rupture of Graafian follicle and release of secondary oocyte.

1
Luteal Phase (Days 15-28):

* Ruptured follicle forms Corpus Luteum (under LH). * Corpus Luteum secretes large amounts of Progesterone and some Estrogen. * Progesterone prepares endometrium for implantation (secretory phase). * If no pregnancy, Corpus Luteum degenerates into Corpus Albicans (around Day 26-28).

III. Uterine Cycle:

1
Menstrual Phase (Days 1-5):

* Caused by sharp drop in Progesterone and Estrogen (due to corpus luteum degeneration). * Endometrial functional layer sheds (menstruation).

1
Proliferative Phase (Days 6-14):

* Driven by Estrogen from developing follicles. * Endometrial lining regenerates and thickens.

1
Secretory Phase (Days 15-28):

* Driven by Progesterone (and Estrogen) from corpus luteum. * Endometrium becomes highly vascularized, glandular, and secretory, ready for implantation.

IV. Key Points for NEET:

Menarche: — Onset of menstruation.
Menopause: — Cessation of menstruation.
Fertile Window: — Around ovulation (sperm viable for days, egg for 12-24 hrs).
Luteal Phase: — Relatively constant 14 days. Ovulation day = Cycle Length - 14.
Hormone Graphs: — Be able to interpret graphs showing FSH, LH, Estrogen, Progesterone levels and correlate with cycle events.

Vyyuha Quick Recall

To remember the key hormones and their primary roles in the menstrual cycle, think:

Follicles Start Here (FSH for Follicle growth) Lots of Hormone, Ovulation! (LH surge for Ovulation) Every Structure Thickens (Estrogen for Endometrial thickening/Proliferation) Prepares Ready Ovum's Ground (Progesterone for preparing uterine ground for implantation)