Menstrual Cycle
மாதவிடாய் சுழற்சி
The menstrual cycle is not merely bleeding; it is a complex hormonal orchestration designed for potential implantation. If you confuse the follicular phase with the luteal phase, you lose marks on the most predictable question in the paper. Precision in timing is the difference between a doctor and a dropout.
The idea
The cycle begins with the menstrual phase, followed by the follicular phase where primary follicles grow into Graafian follicles under FSH influence. Ovulation occurs mid-cycle, around day 14, triggered by an LH surge. The luteal phase follows, dominated by progesterone secretion from the corpus luteum to maintain the endometrium for implantation.
Why it matters
To prepare the uterus for potential implantation of a fertilized ovum and to reset the system if fertilization does not occur.
NEET lens
NEET focuses heavily on the hormone concentration graphs and the specific days of LH surge. Memorize the exact roles of FSH, LH, Estrogen, and Progesterone in each phase. Do not confuse the secretory phase with the luteal phase; they are the same. Expect at least one question matching hormones to their respective cycle phases.
Clinical significance
Failure of the corpus luteum to maintain progesterone levels leads to premature endometrial shedding and potential infertility. Amenorrhea, the absence of menstruation, is a critical clinical indicator of hormonal imbalances or underlying systemic pathologies. Understanding this cycle is fundamental to diagnosing PCOS and endocrine disorders.
Introduction
The menstrual cycle is the approximately 28-day hormonal cycle in females that prepares the uterus for pregnancy, involving cyclical changes in ovaries (follicular development, ovulation, luteal phase) and uterus (endometrial thickening and shedding), regulated by FSH, LH, estrogen, and progesterone.
Core concept
Phases (28-day cycle): (1) Menstrual phase (Day 1–5): Endometrium sheds (if no fertilization). Low estrogen, low progesterone. FSH begins to rise → stimulates follicle development. (2) Follicular/Proliferative phase (Day 6–13): FSH → follicle development → estrogen rises. Estrogen → endometrium proliferates (thickens). LH surge triggered by high estrogen (positive feedback). (3) Ovulatory phase (Day 14): LH surge → ovulation (Graafian follicle ruptures → secondary oocyte released). (4) Luteal/Secretory phase (Day 15–28): Ruptured follicle → corpus luteum → secretes progesterone + estrogen. Endometrium becomes secretory (glands develop). If no fertilization → corpus luteum degenerates (Day 26-28) → progesterone/estrogen fall → endometrium sheds → new cycle. Menarche = first menstrual cycle. Menopause = cessation (~45–50 years).
Go deeper
Positive feedback at LH surge: rising estrogen (from Graafian follicle) switches to positive feedback on LH release (unlike usual negative feedback) → massive LH surge → ovulation. This is the ONLY positive feedback loop in the reproductive axis. After ovulation, corpus luteum produces progesterone which provides negative feedback → FSH and LH fall.
From Class 10
Every month, a woman's reproductive system goes through a cycle of changes to prepare for a possible pregnancy. If pregnancy does not occur, the lining sheds as menstruation. This monthly cycle is the menstrual cycle.