Menstrual Cycle

NCERT grounding

NCERT Class 12 Biology, Chapter 2 (Human Reproduction), Section 2.4 is the canonical syllabus anchor for this subtopic. It states that the reproductive cycle in female primates — including monkeys, apes and humans — is called the menstrual cycle, that menstruation is repeated at an average interval of about 28 to 29 days, and that one ovum is released per cycle. NIOS Biology Lesson 21 (Reproduction and Population Control), Section 21.2.3, supplements this with the puberty-to-menopause age window of 11–13 to 45–50 years and a parallel description of the four phases.

Four phases of the cycle

A typical human menstrual cycle is laid out on a ~28-day timeline that starts on the first day of menstrual bleeding (day 1) and ends on the day before the next bleed. NCERT presents the cycle as a four-phase sequence: the menstrual phase, the follicular phase, the ovulatory phase and the luteal phase. The clinical literature often collapses follicular and ovulatory into a single "follicular" half (days 1–14) and renames luteal as "secretory" (days 15–28); both vocabularies are valid and both appear in NEET.

The cycle is regulated by a four-tier hierarchy: hypothalamic GnRH drives anterior-pituitary FSH and LH, which in turn drive ovarian oestrogen and progesterone. The ovarian steroids close the loop with feedback (mostly negative, briefly positive). On the target organ side, the endometrium of the uterus responds to oestrogen by proliferating and to progesterone by becoming secretory and receptive to implantation.

Phase 1 — Menstrual phase (days 1–5)

The cycle begins, by convention, on the first day of menstrual flow. Menstrual flow results from the breakdown of the endometrial lining of the uterus and its associated blood vessels, producing a liquid that exits through the cervix and vagina. NCERT specifies the duration as 3 to 5 days; NIOS gives 3 to 4 days. About 30–80 mL of blood, sloughed endometrium and cervical mucus is shed.

Crucially, menstruation only occurs if the released ovum of the previous cycle was not fertilised. The withdrawal of progesterone after the luteal phase causes spiral arteries in the endometrium to constrict, the tissue becomes ischaemic, and the functional layer (stratum functionalis) sloughs off, leaving the basal layer (stratum basalis) intact for regeneration. Lack of menstruation may therefore be indicative of pregnancy — although NCERT cautions that it may also be caused by stress, poor health or other underlying causes.

Phase 2 — Follicular / proliferative phase (days 6–13)

As the menstrual phase ends, the follicular phase begins in the ovary and a parallel proliferative phase begins in the uterus. NCERT describes this as the phase "during which the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation."

FSH from the anterior pituitary recruits a cohort of antral follicles; one becomes dominant and progresses through the secondary follicle stage to a tertiary, fluid-filled Graafian follicle. Granulosa cells of the growing follicle, stimulated by FSH and using androgens supplied by theca interna cells, synthesise oestrogen (chiefly 17β-oestradiol). NCERT specifies that the gonadotropins LH and FSH "increase gradually during the follicular phase" — a fact that has appeared verbatim in NEET 2016 as a trap.

Rising oestrogen drives the uterine endometrium to proliferate: glands lengthen, stromal cells multiply and spiral arteries extend. By the end of the follicular phase, the endometrium is 3–5 mm thick, oestrogen has peaked, and the dominant follicle has become preovulatory.

Phase 3 — Ovulatory phase (around day 14)

Once oestrogen crosses a threshold concentration sustained for ~48 hours, it switches from exerting negative feedback to exerting positive feedback on the hypothalamus and anterior pituitary. NCERT describes the consequence directly: "Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation)."

The LH surge is the single most NEET-tested trigger in this chapter. It does not cause meiosis I (that finishes inside the follicle just before ovulation), nor does it cause fertilisation. It mechanically ruptures the Graafian follicle wall — through prostaglandin and proteolytic enzyme release — and the secondary oocyte (arrested in metaphase II) is swept into the fallopian tube by the fimbriae of the infundibulum.

Phase 4 — Luteal / secretory phase (days 15–28)

After ovulation, the ruptured Graafian follicle is not discarded. Under continuing LH influence, its granulosa and theca interna cells luteinise and form the corpus luteum, a transient endocrine gland that secretes large amounts of progesterone along with some oestrogen and inhibin. NCERT is unambiguous: "The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium."

Progesterone reshapes the proliferative endometrium into a secretory endometrium: glands coil and secrete glycogen-rich nutrient fluid, spiral arteries thicken, and the stroma becomes oedematous. This receptive state is required for implantation of the blastocyst at around day 21–22 if fertilisation has occurred.

Hormone curves & feedback

The four hormones tracked across the cycle — FSH, LH, oestrogen and progesterone — have distinctive temporal profiles that NEET expects students to read from a graph. The follicular phase shows gradually rising FSH and LH with oestrogen climbing steeply at the end. Mid-cycle delivers a sharp LH spike (the surge) with a smaller FSH peak; oestrogen briefly dips after ovulation. The luteal phase is dominated by a broad progesterone dome with a smaller oestrogen secondary peak, both falling sharply if luteolysis occurs.

The feedback architecture flips polarity exactly once per cycle. For most of the follicular phase, oestrogen and inhibin exert negative feedback on the hypothalamus (suppressing GnRH pulses) and on the pituitary (suppressing FSH preferentially). When oestrogen exceeds approximately 200 pg/mL for ≥48 hours, the feedback sign flips to positive — GnRH pulse frequency rises and the pituitary releases its stored LH as a surge. After ovulation, progesterone restores strong negative feedback, holding FSH and LH low for the entire luteal phase. The NEET 2016 stem "Change in GnRH pulse frequency in females is controlled by circulating levels of …" is answered by this logic: oestrogen and progesterone.

Menarche, menopause & cycle boundaries

NCERT and NIOS bracket the reproductive phase of a human female between two named events. The first menstruation, called menarche, begins at puberty (NIOS: 11–13 years). The permanent cessation of menstrual cycles is called menopause and occurs at approximately 50 years of age (NIOS: 45–50). NCERT phrases the boundary explicitly: cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause.

Menstrual vs. oestrous cycle

NCERT contrasts the menstrual cycle (only in primates — monkeys, apes, humans) with the oestrous cycle seen in non-primate placental mammals (cows, dogs, rats). The two are not synonyms. In a menstrual cycle, an unfertilised endometrium is shed externally as menstrual flow; in an oestrous cycle, the endometrium is reabsorbed and there is no external bleeding. The female is sexually receptive only during oestrus ("heat"), whereas menstrual-cycle primates are receptive throughout the cycle. The NEET 2023 stem "In non-primate mammals cyclical changes during reproduction are called oestrus cycle" tests exactly this distinction.

Worked examples

Common confusion & NEET traps