Reproductive Endocrinology
Menstrual Cycle
The 28-day hormonal rhythm of reproduction
The menstrual cycle is a complex 28-day hormonal rhythm that prepares the female body for potential pregnancy, driven by a precise coordination between the brain and the ovaries. Around day 14, a dramatic surge in Luteinizing Hormone (LH) triggers ovulation—the release of a mature egg—marking the most critical threshold of the cycle. If fertilization does not occur, the drop in progesterone and estrogen triggers the shedding of the endometrium, resetting the system for the next cycle.
- Median Length28 days (Range 21–35)
- OvulationDay 14 (approximate)
- LH Surge~36 hours before ovulation
- Luteal PhaseStrictly ~14 days
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How it works
The cycle is divided into two main phases. The Follicular Phase (Days 1–14) begins with menstruation. FSH from the brain stimulates follicles in the ovary to grow, which in turn produce estrogen. High estrogen eventually triggers a massive spike in LH. This 'LH Surge' causes the dominant follicle to rupture and release an egg (ovulation). The Luteal Phase (Days 15–28) begins as the empty follicle becomes the 'corpus luteum,' producing progesterone to thicken the uterine lining in anticipation of an embryo.
The Hormonal Feedback
The cycle is a masterclass in feedback loops. Early on, estrogen provides 'negative feedback' to keep FSH and LH low. But at a specific concentration and duration, estrogen flips to 'positive feedback,' causing the brain to dump LH all at once. If no pregnancy occurs, the corpus luteum dies, progesterone levels crash, and the lack of hormonal support causes the uterine lining to break down—resulting in period bleeding (menstruation).
Common pitfalls
- Assuming Day 14 is universal: Ovulation can vary widely; however, the time *after* ovulation (luteal phase) is almost always exactly 14 days.
- Ignoring the 'Fertile Window': Sperm can survive for ~5 days, meaning pregnancy can result from sex days before ovulation actually occurs.
- Mistaking spotting for a period: 'Implantation bleeding' can sometimes be confused with a light period, leading to miscalculated pregnancy dates.
| Feature | Follicular Phase | Luteal Phase |
|---|---|---|
| Timing | Days 1 to 14 | Days 15 to 28 |
| Dominant Hormone | Estrogen | Progesterone |
| Ovarian Event | Follicle development | Corpus luteum activity |
| Uterine Event | Lining shedding + regrowth | Lining stabilization / secretion |
Frequently asked questions
When am I most fertile?
During the 'fertile window,' which includes the day of ovulation and the 5 days leading up to it, as sperm can survive in the reproductive tract for several days.
What causes the LH surge?
A high level of estrogen sustained for about two days triggers the pituitary gland to release a massive amount of Luteinizing Hormone (LH) all at once.
Why do periods happen?
If no egg is fertilized, the corpus luteum stops producing progesterone. This sudden drop in hormones tells the uterus that no pregnancy has occurred, causing it to shed its lining.
How does birth control work?
Most hormonal contraceptives provide a steady level of estrogen and/or progestin that prevents the LH surge, thereby stopping ovulation from ever happening.
What is the 'Corpus Luteum'?
It is a temporary gland formed in the ovary from the follicle that just released an egg. Its main job is to produce progesterone to support a potential pregnancy.