15.3 Menstrual Cycle
The menstrual cycle is a repeating cycle involving the production of a secondary oocyte and the thickening of the endometrial wall to prepare for embryo implantation. It is precisely regulated by hormones released by the pituitary gland and the ovaries. If fertilisation does not occur, the endometrium sheds as menstruation.
Learning Objectives
- State the definition of the menstrual cycle and menstruation.
- Identify the hormones involved in the menstrual cycle (FSH, LH, oestrogen, progesterone) and state their respective glands and functions.
- Explain the changes in hormone levels, follicle development, and endometrial thickness across the four stages of the menstrual cycle (Day 0–5, Day 6–14, Day 15–21, Day 22–28).
- Explain the roles of hormones in pregnancy and the hormonal cause of miscarriage.
- Describe premenstrual syndrome (PMS) and its symptoms.
- Describe menopause and its associated symptoms.
Hormonal and Endometrial Changes in the Menstrual Cycle
A 28-day cycle showing synchronous changes in pituitary hormones, ovarian hormones, follicle development and endometrial thickness.
Day 0–5
Menstruation and follicle growth initiation. Low hormone levels cause the thickened endometrium to shed.
Day 6–14
Follicle maturation and ovulation. Oestrogen peaks on day 12, LH surges on day 13, ovulation on day 14.
Day 15–21
Luteal phase. Corpus luteum secretes oestrogen and progesterone to thicken the endometrium.
Day 22–28
Degeneration or continuation. Corpus luteum degenerates if no fertilisation; pregnancy continues if fertilisation occurs.
Short Explanation
Definition
Menstrual cycle — The menstrual cycle involves the production of a secondary oocyte and the thickening of the endometrial wall throughout one cycle. During this cycle, the endometrium becomes soft, thick, and rich with blood vessels to prepare for embryo implantation. If fertilisation does not take place, the secondary oocyte dies and the endometrium wall sheds, leading to bleeding known as menstruation.
Pituitary Gland Hormones
FSH stimulates follicle growth in the ovary and stimulates the release of oestrogen. LH stimulates ovulation, causes the formation of the corpus luteum, and stimulates the release of progesterone.
Ovarian Hormones
Oestrogen repairs and stimulates the thickening of the endometrium, stimulates follicle growth until it matures, and stimulates FSH and LH release prior to ovulation. Progesterone stimulates the thickening of the endometrium, making it thick, folded, and rich in blood vessels, and stops the release of FSH and LH.
Pregnancy and Miscarriage
The corpus luteum continues to produce oestrogen and progesterone for up to 3 to 4 months of pregnancy. Thereafter, the corpus luteum degenerates, and the production of oestrogen and progesterone is taken over by the placenta until birth. An imbalance of progesterone and oestrogen levels may lead to a miscarriage, due to a decrease in progesterone level causing the uterus to shrink.
Premenstrual Syndrome and Menopause
Premenstrual syndrome (PMS) symptoms appear prior to a menstrual cycle, usually manifesting between 7 to 14 days before the first day of menstruation. Menopause naturally occurs between the ages of 46 to 50 years old, when ovulation and menstruation stop.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What triggers ovulation on day 14 of the menstrual cycle?
What happens to the corpus luteum if fertilisation does not take place?
Why do menstruation and ovulation not occur during pregnancy?
Common Mistakes
- Confusing the peak day of oestrogen (day 12) with the peak day of LH (day 13) and the day of ovulation (day 14).
- Stating that the placenta secretes progesterone immediately after fertilisation, whereas the corpus luteum secretes it for the first 3 to 4 months before the placenta takes over.
- Thinking FSH causes ovulation, when in fact LH stimulates ovulation.
- Confusing premenstrual syndrome (occurs 7–14 days before menstruation due to hormone imbalance) with menopause (permanent cessation of menstruation at 46–50 years old).
Concept Misunderstandings
High levels of oestrogen always inhibit FSH and LH.
Low levels of oestrogen inhibit FSH and LH via negative feedback, but a high level of oestrogen (peaking around day 12) stimulates GnRH, FSH, and LH release via positive feedback.
The endometrium thickens solely because of progesterone.
Oestrogen initially repairs and stimulates the thickening of the endometrium (days 6–14), while progesterone further thickens, folds, and enriches it with blood vessels (days 15–21).
Summary
- Menstruation marks Day 1 of the cycle, triggered by a decline in oestrogen and progesterone.
- FSH stimulates follicle growth; growing follicles secrete oestrogen to repair the endometrium.
- An LH peak on day 13 triggers ovulation of a secondary oocyte on day 14 and forms the corpus luteum.
- The corpus luteum secretes progesterone and oestrogen to maintain the endometrium and inhibit FSH/LH.
- Degeneration of the corpus luteum restarts the cycle unless pregnancy occurs, in which case the corpus luteum maintains hormone levels for 3–4 months until the placenta takes over.
Short Activity
Objective: identify the hormones, stages and key events of the menstrual cycle.
A. Objective Quiz
Which hormone reaches its peak level on day 13, directly stimulating ovulation on day 14?
What is the primary cause of a miscarriage related to hormonal imbalance during pregnancy?
B. Fill in the Blanks
During pregnancy, the corpus luteum produces oestrogen and progesterone for three to four months before this function is taken over by the until birth.
Menopause occurs naturally when ovulation and menstruation stop, usually between the ages of to 50 years old.
C. Matching / Drag and Drop
Match each hormone to its correct primary function in the menstrual cycle. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Hormone to Its Function
Write the correct hormone next to each function.
| Function | Hormone |
|---|---|
| 1. Stimulates follicle growth in the ovary and stimulates the release of oestrogen | |
| 2. Stimulates ovulation and causes the formation of the corpus luteum | |
| 3. Repairs the endometrium and stimulates FSH and LH release prior to ovulation | |
| 4. Makes endometrium thick, folded, and rich in blood vessels, and inhibits FSH and LH |
Answer choices: FSH • LH • Oestrogen • Progesterone
