The menstrual cycle and its hormones

A cycle that repeats

In a woman, the ovary and the uterus change together in a regular way. The first menstruation at puberty is menarche. After it, menstruation comes at an average interval of about 28 or 29 days. The events from one menstruation to the next are the menstrual cycle. Ovulation takes place in the middle of the cycle, so one ovum is released in each cycle. The cycles stop at about 50 years of age, which is menopause.

The graph has four parts. Read it from top to bottom, and from left to right.

The four phases

1. Menstrual phase (days 1 to 5, lasting 3 to 5 days). The lining of the uterus, the endometrium, and its small blood vessels break down. The result is a fluid that comes out through the vagina. This is menstruation.
2. Follicular phase (day 6 to about day 13). Primary follicles grow in the ovary, and one becomes a mature Graafian follicle. At the same time the endometrium builds up again. This is called proliferation.
3. Ovulation (about day 14). The Graafian follicle bursts and the ovum is released.
4. Luteal phase (about day 15 to day 28). The rest of the follicle becomes the corpus luteum. The endometrium is made ready for the embryo.

Menstruation takes place only if the ovum is not fertilised. If a woman misses a menstruation, it may be a sign of pregnancy. It can also be caused by stress, poor health or other causes.

The hormones

Two hormones of the pituitary, the gonadotropins LH and FSH, and two hormones of the ovary, oestrogen and progesterone, control the cycle.

• During the follicular phase, the secretion of LH and FSH rises slowly. They make the follicles grow, and the growing follicles secrete oestrogen.
• Both LH and FSH reach a peak in the middle of the cycle, about day 14. The rapid rise of LH to its maximum is the LH surge. It makes the Graafian follicle burst. This is ovulation.
• In the luteal phase, the corpus luteum secretes large amounts of progesterone. Progesterone keeps the endometrium ready for implantation and for the events of pregnancy.
• If there is no fertilisation, the corpus luteum degenerates. The progesterone falls, the endometrium falls apart, and menstruation begins. A new cycle starts.
• During pregnancy the cycles stop, and there is no menstruation.

This part is not in the NEET syllabus, but A-Level and IB ask it. A rising oestrogen level first lowers the LH and FSH. When it stays high for a few days, it has the opposite effect and causes the LH surge. This is positive feedback. In the luteal phase, progesterone and oestrogen lower the LH and FSH again, so no new follicle ripens.

Reading the length of the phases

Cycles do not always last 28 days. What stays nearly the same is the time between ovulation and the next menstruation: about 14 days. The time before ovulation changes. In a cycle of 35 days, ovulation is on day 21. In a cycle of 26 days, it is on day 12.

You should find that a longer cycle moves the peaks to the right, but the progesterone peak always follows the LH peak by about a week.

Try it in Python

The program prints the phase of every day of a cycle of 28 days.

Practise with new numbers

Common mistakes

• Saying that ovulation is always on day 14. It is on day 14 only in a cycle of 28 days.
• Saying that progesterone makes the follicle grow. The gonadotropins and the oestrogen do this. Progesterone comes from the corpus luteum.
• Saying that menstruation is caused by a high level of progesterone. It is caused by the fall of progesterone, after the corpus luteum degenerates.
• Mixing the phases of the ovary and of the uterus. The ovary has the follicular and luteal phases. The uterus has the menstrual, proliferative and secretory phases.
• Saying that the LH surge makes the follicle grow. It makes the mature follicle burst.

Remember these

• Menarche at puberty, menopause at about 50 years. Average cycle: 28 or 29 days. One ovum in each cycle.
• Menstrual phase 3 to 5 days, then the follicular phase, ovulation about day 14, then the luteal phase.
• The LH surge in the middle of the cycle causes ovulation. The corpus luteum makes progesterone.
• No fertilisation: the corpus luteum degenerates, the progesterone falls, and menstruation starts.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How long is the average menstrual cycle, and on which day does ovulation usually take place?

2. What causes ovulation?

3. What does the corpus luteum secrete, and why is it needed?

4. Why does menstruation begin when the ovum is not fertilised?

5. A woman has cycles of 30 days. When does she ovulate?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 27, The Reproductive System.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 81.