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The Menstrual Cycle in Two Acts

Follicular phase, then luteal phase, with ovulation as the pivot between them. Follow one egg's month — what rises, what falls, and what the uterine lining is doing the whole time.

Two acts and an intermission

The menstrual cycle is easiest to learn as a play in two acts with a single dramatic moment in the middle. Act one is the follicular phase: a follicle grows and estrogen rises. The intermission is ovulation: the egg is released. Act two is the luteal phase: the leftover follicle becomes a structure that pours out progesterone. If no pregnancy occurs, the curtain falls and the lining sheds as a period — day 1 of the next cycle. A “textbook” cycle is about 28 days, but anywhere from roughly 24 to 35 is normal.

Act one: the follicular phase

  1. Early on, FSH nudges a cohort of small follicles to start growing. They compete, and usually one — the dominant follicle — pulls ahead while the others wither.
  2. The growing follicle's granulosa cells make more and more estradiol. Blood estrogen climbs steadily through the first two weeks.
  3. That rising estrogen tells the uterus to rebuild: the lining thickens and grows new glands and blood vessels. This is the “proliferative” stage — estrogen the builder at work.

Act two: the luteal phase

After ovulation (the full mechanism is guide 4), the emptied follicle doesn't just vanish. It collapses and transforms into the corpus luteum — Latin for “yellow body” — a temporary gland that becomes the cycle's main source of progesterone. Now progesterone the holder takes over: it converts the estrogen-built lining into a stable, secretory, nourishing surface, ready in case an embryo arrives. Body temperature ticks up slightly under progesterone's influence, which is why charted morning temperatures show a small rise after ovulation.

The corpus luteum has a built-in clock. If no pregnancy signals it to keep going, it self-destructs after about two weeks. Progesterone and estrogen both crash, the now-unsupported lining breaks down and sheds, and bleeding begins. That bleed is day 1 — and the cycle starts over. (If pregnancy *does* occur, an embryonic hormone rescues the corpus luteum; that is a story for the pregnancy track.)

ONE CYCLE AT A GLANCE (~28 days)

Day:   1 ----- 7 ----- 14 ----- 21 ----- 28 -> 1
       |  follicular   | |  luteal       |
Phase: [period][--building--]X[--holding----][period]
                            ^
                       OVULATION (~mid)

Estrogen:  low -> rising ---> PEAK -> dip -> 2nd hump -> fall
Progest.:  low -------- low --|-- rising -> high -> fall
Uterus:    shed -> proliferative -|- secretory -> shed

Luteal length ~14 d is fixed; follicular length varies.
Estrogen leads the first half; progesterone (from the corpus luteum) defines the second.