📅 Reproductive System Lesson

MOSL: four menstrual cycle phases

A 28-day cycle divided into four phases, each dominated by a different hormone — understanding the hormone driving each phase explains everything else about the cycle.

Menstrual

Ovulatory

Secretory

Luteal

📖 Full Breakdown

Four phases across a standard 28-day cycle, each with a dominant hormone

Ovulation marks the hinge point of the entire cycle — everything before it is preparation, everything after it depends on whether fertilization occurred.

Menstrual phase (Days 1-5)

Endometrium sheds, FSH rises

The endometrium built up during the previous cycle is shed as menstrual flow, while FSH begins rising to stimulate the next follicle's development.

Proliferative/Follicular phase (Days 6-13)

Estrogen rises, endometrium rebuilds

A dominant follicle matures under FSH influence while rising estrogen rebuilds the endometrial lining.

Ovulation (Day 14)

LH surge triggers egg release

This is the single most fertile point of the cycle, triggered by a sharp spike in LH.

Luteal/Secretory phase (Days 15-28)

Corpus luteum produces progesterone

The ruptured follicle becomes the corpus luteum, producing progesterone to prepare the endometrium for potential implantation. If no pregnancy occurs, the corpus luteum degenerates, progesterone drops, and menstruation begins again.

🩺 Clinical / Exam Application

A woman tracking her cycle for fertility purposes notices a slight rise in basal body temperature a day or two after her predicted ovulation date. This temperature shift happens because progesterone, released by the newly-formed corpus luteum during the luteal phase, has a mild thermogenic (heat-generating) effect on the body. Because this hormone is produced specifically AFTER ovulation (not before), tracking this exact temperature shift lets her confirm ovulation already occurred, retrospectively — a practical, everyday application of understanding exactly which hormone dominates which specific phase.

⚠️ Exam Alert

A frequently tested clinical connection: progesterone specifically maintains early pregnancy (before the placenta takes over hormone production), and its source — the corpus luteum, formed only after ovulation — is why the luteal phase is so critical to early pregnancy viability if fertilization occurs.

🚧 Common Trap

Don't assume "follicular phase" and "proliferative phase" are two different things happening at two different times. These are simply two different names for the SAME phase (days 6-13) — describing the ovarian event (follicle developing) and the uterine event (endometrium proliferating) that happen simultaneously.

✅ Quick Check

Why does progesterone only appear in significant amounts AFTER ovulation, rather than throughout the entire cycle?

📝 Exam Prep

Common Exam Questions

❓ What are the four phases of the menstrual cycle?

✅ Menstrual (days 1-5, endometrium shed, FSH rises), Proliferative/Follicular (days 6-13, estrogen rises, endometrium rebuilds), Ovulation (day 14, LH surge triggers egg release), and Luteal/Secretory (days 15-28, corpus luteum produces progesterone).

❓ What happens if pregnancy does not occur after ovulation?

✅ The corpus luteum degenerates, progesterone levels drop, and the endometrium is shed, beginning menstruation and restarting the cycle.