Explain in detail about Menstrual Cycle

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Menstrual cycle

The menstrual cycle is the recurring, hormone-controlled sequence that prepares the ovary to release an egg and prepares the uterus for a possible pregnancy. If pregnancy does not occur, the uterine lining is shed as menstrual bleeding.

Day 1 is the first day of full menstrual bleeding.
A commonly taught cycle is 28 days, but healthy cycles vary between people and from cycle to cycle.
Two coordinated cycles occur at the same time:
1. Ovarian cycle: follicle development, ovulation, and corpus luteum formation.
2. Uterine or endometrial cycle: menstruation, proliferation, and secretion.

The cycle is regulated by the hypothalamic-pituitary-ovarian axis:
Hypothalamus (GnRH) → anterior pituitary (FSH and LH) → ovary (estrogen, progesterone, inhibin) → uterus and feedback to brain/pituitary.

Costanzo Physiology, 7th ed., p. 477; Berek & Novak's Gynecology, p. 264.

Hormonal control of the menstrual cycle: hypothalamus, pituitary, and ovary

1. Main hormones and their roles

Hormone	Main source	Main function
GnRH	Hypothalamus	Released in pulses; stimulates pituitary release of FSH and LH
FSH	Anterior pituitary	Recruits and supports growth of ovarian follicles; stimulates granulosa cells and estrogen production
LH	Anterior pituitary	Triggers ovulation; promotes corpus luteum formation
Estrogen, mainly estradiol	Growing ovarian follicles	Builds the endometrium; usually inhibits FSH/LH, but sustained high levels before ovulation trigger positive feedback and the LH surge
Progesterone	Corpus luteum after ovulation	Converts endometrium to a secretory, implantation-ready state; stabilizes it and suppresses further FSH/LH release
Inhibin	Granulosa cells and corpus luteum	Suppresses FSH, helping select a dominant follicle

GnRH must be pulsatile. Continuous GnRH exposure eventually suppresses pituitary FSH and LH secretion.

2. Ovarian cycle

The ovarian cycle has three parts:

Follicular phase
Ovulation
Luteal phase

A. Follicular phase: from menstruation to ovulation

This phase begins on day 1 of bleeding and ends at ovulation. Its length is the most variable part of the cycle.

Early follicular phase

At the end of the preceding cycle, the corpus luteum degenerates. Therefore:

Progesterone and estrogen fall
Their negative feedback on the pituitary is removed
FSH rises modestly
Several ovarian follicles begin growing

Each developing follicle contains an oocyte. FSH acts primarily on granulosa cells, stimulating follicular growth and aromatase activity. Aromatase converts androgens into estradiol.

Dominant follicle selection

Usually, one follicle becomes the dominant follicle. It is selected because it responds best to FSH and produces increasing estradiol. The other recruited follicles undergo atresia.

As estradiol rises:

It promotes endometrial growth.
It reduces FSH through negative feedback, which helps prevent maturation of multiple follicles.
Inhibin B from granulosa cells also reduces FSH.

B. Ovulation: release of the oocyte

Near the middle of the cycle, sustained high estradiol from the mature dominant follicle switches from its usual negative feedback to positive feedback on the hypothalamus and pituitary.

This produces:

A large LH surge
A smaller FSH surge

The LH surge causes:

Completion of the first meiotic division by the primary oocyte
Rupture of the mature follicle
Release of the secondary oocyte
Beginning of luteinization, which turns the ruptured follicle into the corpus luteum

In a textbook 28-day cycle, ovulation is often placed around day 14, but it does not always occur on day 14. It normally occurs about 12 to 14 days before the next period, because the luteal phase is relatively stable while the follicular phase varies. The onset of the LH surge typically precedes ovulation by roughly 34 to 36 hours, according to Endotext's ovulation review.

C. Luteal phase: from ovulation to next menstruation

After ovulation, LH transforms the ruptured follicle into the corpus luteum.

The corpus luteum secretes:

Progesterone, predominantly
Estrogen
Inhibin A

These hormones exert negative feedback on GnRH, FSH, and LH. This prevents another ovulation within the same cycle.

If fertilization and implantation do not occur:

The corpus luteum survives for about 12 to 14 days, then regresses.
Progesterone and estrogen concentrations decline sharply.
The endometrium loses hormonal support.
Menstruation begins.
FSH begins to rise again, initiating the next cycle.

If pregnancy occurs, the early embryo produces human chorionic gonadotropin (hCG), which maintains the corpus luteum until the placenta can produce sufficient progesterone and estrogen.

3. Uterine or endometrial cycle

The endometrium is the inner lining of the uterus. Its changes are synchronized with ovarian hormone secretion.

Endometrial phase	Approximate timing in a 28-day cycle	Dominant hormone/event
Menstrual phase	Days 1-4 or 5	Withdrawal of progesterone and estrogen
Proliferative phase	After menses to ovulation	Estrogen
Secretory phase	After ovulation to menses	Progesterone

A. Menstrual phase

Menstruation occurs because the corpus luteum regresses and progesterone and estrogen fall.

Progesterone withdrawal causes:

Constriction of the endometrial spiral arteries
Local ischemia and tissue breakdown
Inflammatory mediator and prostaglandin release
Breakdown and shedding of the superficial functional layer of endometrium

The basal layer remains intact. It regenerates the endometrium during the next proliferative phase.

Menstrual blood contains blood, endometrial tissue, cervical mucus, and vaginal secretions. Normal menstrual loss is not simply “loss of blood.”

B. Proliferative phase

This occurs after menstruation and before ovulation. It overlaps with the ovarian follicular phase.

Rising estradiol from the developing follicle causes:

Regrowth and thickening of the functional endometrium
Proliferation of endometrial stromal cells
Lengthening of endometrial glands
Growth of spiral arteries
Production of thinner, clearer, more watery cervical mucus

This watery cervical mucus near ovulation makes sperm passage through the cervix easier.

C. Secretory phase

This begins after ovulation and overlaps with the ovarian luteal phase.

Progesterone from the corpus luteum changes the estrogen-primed endometrium into one suitable for implantation:

Glands become coiled and secrete nutrient-rich material, including glycogen
The endometrium becomes more edematous and vascular
Stromal cells begin decidual changes
Cervical mucus becomes thicker and less penetrable to sperm
Basal body temperature rises slightly, often by about 0.3 to 0.5°C, due to progesterone

The secretory endometrium is optimally receptive to implantation about 6 to 10 days after ovulation.

The correspondence between ovarian and uterine events is summarized in StatPearls' physiology review.

4. Hormonal pattern through the cycle

A simple way to remember the pattern:

Beginning of cycle: estrogen and progesterone are low; FSH rises.
Follicular phase: estrogen progressively rises as the dominant follicle matures.
Just before ovulation: prolonged high estrogen causes the LH surge.
After ovulation: progesterone rises because of the corpus luteum.
Late luteal phase: progesterone and estrogen fall if pregnancy has not occurred, triggering menstruation.

Feedback mechanisms

Negative feedback

For most of the cycle:

Moderate estrogen inhibits GnRH, FSH, and LH.
Progesterone inhibits GnRH, FSH, and LH.
Inhibin suppresses FSH.

Positive feedback

Immediately before ovulation:

Sustained high estradiol changes to positive feedback.
This generates the LH surge required for ovulation.

This switch is the defining endocrine event of the mid-cycle period.
Costanzo Physiology, 7th ed., p. 477.

5. Relationship to fertility

The fertile window includes the days before ovulation and the day of ovulation:

Sperm can survive in the female reproductive tract for several days under favorable cervical mucus conditions.
The oocyte remains capable of fertilization for only about 12 to 24 hours after ovulation.

Therefore, pregnancy can result from intercourse in the days before ovulation, not only on the day of ovulation.

Common possible signs of ovulation include:

Clear, stretchy, slippery cervical mucus
Mild one-sided pelvic discomfort in some people, called mittelschmerz
A small sustained rise in basal body temperature after ovulation
Positive urinary LH test before ovulation

These signs can help estimate ovulation but do not guarantee it.

6. Premenstrual symptoms and cramps

During the late luteal phase, hormonal changes may contribute to:

Breast tenderness
Bloating
Acne
Fatigue
Food cravings
Mood changes
Headache

Menstrual cramps, or primary dysmenorrhea, are mainly related to endometrial prostaglandins, particularly prostaglandin F2α. These increase uterine contractions and can temporarily reduce uterine blood flow.

7. When menstrual patterns may need medical assessment

Clinical review is appropriate for situations such as:

Very heavy bleeding, such as soaking pads or tampons hourly for several hours
Bleeding lasting over about 7 days repeatedly
Periods consistently less than 21 days or more than 35 days apart in adults
No period for 3 months without an obvious explanation
Severe pain, new pelvic pain, fainting, or symptoms of anemia
Possible pregnancy with bleeding or pain
New irregular bleeding after previously regular cycles
Bleeding after sex or after menopause

Common contributors to abnormal cycles include pregnancy, stress, weight changes, intense exercise, polycystic ovary syndrome, thyroid disease, hyperprolactinemia, perimenopause, certain medicines, and structural uterine conditions such as fibroids.

One-line summary

FSH develops follicles → estrogen proliferates endometrium → sustained high estrogen causes the LH surge → LH triggers ovulation → corpus luteum produces progesterone → progesterone makes the endometrium secretory → hormone withdrawal causes menstruation if there is no pregnancy.

Explain the menstrual cycle with a simple diagram

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Menstrual Cycle: Simple Diagram

DAY:       1          5          14                    28
           |----------|-----------|---------------------|
           
OVARY:     Follicles grow       Ovulation         Corpus luteum
           (FSH, estrogen ↑)    (LH surge)        (progesterone ↑)
                                      |
                                      ↓
UTERUS:    Menstrual bleeding → Endometrium grows → Secretory lining
           (period)               (thickens)        (ready for pregnancy)

IF NO PREGNANCY:
Corpus luteum breaks down → Estrogen and progesterone fall → Period begins again

The three main stages

1. Menstrual phase: about days 1-5

This is the period.
The uterus sheds its inner lining, called the endometrium.
This happens because estrogen and progesterone levels fall.

2. Follicular or proliferative phase: from day 1 to ovulation

The brain releases FSH (follicle-stimulating hormone).
FSH helps several follicles in the ovary begin to grow. Usually one becomes the dominant follicle.
The growing follicle makes increasing estrogen.
Estrogen rebuilds and thickens the uterine lining.

FSH ↑ → Follicle grows → Estrogen ↑ → Uterine lining grows

3. Ovulation: around mid-cycle

When estrogen remains high for a short period, it causes a sudden rise in LH, called the LH surge.
The LH surge causes release of an egg from the ovary. This is ovulation.

High estrogen → LH surge → Egg released (ovulation)

In a 28-day example cycle, ovulation is often around day 14. In real life, the timing can differ. Ovulation usually occurs about 12-14 days before the next period.

4. Luteal or secretory phase: after ovulation

The empty follicle becomes the corpus luteum.
It produces mainly progesterone.
Progesterone changes the thickened uterine lining into a soft, nutrient-rich lining suitable for implantation if pregnancy occurs.

Corpus luteum → Progesterone ↑ → Uterus prepared for implantation

If pregnancy does not occur

No fertilization
       ↓
Corpus luteum degenerates
       ↓
Progesterone and estrogen fall
       ↓
Uterine lining breaks down
       ↓
Menstruation begins: a new cycle starts

Hormone pattern at a glance

FSH:          small rise early in the cycle
Estrogen:     rises before ovulation
LH:           sharp surge at ovulation
Progesterone: rises after ovulation, then falls before the period

Easy sequence to remember:

Period → Follicle grows → Estrogen rises → LH surge → Ovulation
→ Progesterone rises → Uterus prepared → Hormones fall → Period

Costanzo Physiology, 7th ed., p. 477; Berek & Novak's Gynecology, p. 264.

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