Lesson 10 of 12 / Reproduction
The menstrual cycle
How do oestrogen and progesterone connect ovulation with preparation for implantation?
In this lesson: Explain menstruation, ovulation, hormone effects and variation in cycle timing.
About 5 min
The key ideaOestrogen rebuilds the lining; progesterone helps maintain it after ovulation. Falling levels allow menstruation if pregnancy is not established.
The 28-day diagram is an illustrative cycle, not a timetable that applies to every person. Day 1 is the first day of menstruation.
Natural cycles vary. The biologically more fertile phase is around ovulation, including preceding days because sperm can survive; this chart does not identify guaranteed infertile dates.
Explanation
The menstrual cycle alternates changes in the ovaries and uterine lining. Day 1 is the start of menstruation, when part of the lining breaks down and leaves through the cervix and vagina. Natural cycle length varies between people and between cycles.
As a follicle develops in an ovary, increasing oestrogen promotes repair and thickening of the uterine lining. Ovulation is the release of an egg. In a simplified 28-day example it is shown near the middle, but it does not always happen on day 14.
After ovulation, the remaining follicle tissue forms the corpus luteum, which releases progesterone. Progesterone helps maintain the thick, blood-supplied lining in preparation for implantation. Oestrogen also remains involved in supporting the lining.
If pregnancy is not established, progesterone and oestrogen levels fall and the lining is shed, beginning another cycle. If implantation establishes pregnancy, hormonal support maintains the lining instead.
The more fertile phase is around ovulation, including preceding days because sperm can survive for several days. An egg remains viable for a much shorter period after release. At times with no viable egg and no overlap with surviving sperm, fertilisation cannot occur; however, variable timing means a calendar alone cannot identify guaranteed infertile days.
Step by step
- 1
Identify the event
Menstruation removes lining; ovulation releases an egg. They are not the same event.
- 2
Connect hormone and tissue
Oestrogen promotes rebuilding; progesterone supports maintenance after ovulation.
- 3
Qualify the timing
Use the dates only for the example, not as a universal prediction of fertility.
Worked example
A falling hormone level
In a cycle without established pregnancy, progesterone and oestrogen fall near its end. Explain what happens to the uterine lining.
One way to explain it
The hormonal support for maintaining the thick lining decreases. Part of the lining breaks down and is shed during menstruation, beginning the next cycle.
Why this answer works
- Describe an effect on the lining.
- Do not describe menstruation as the release of an egg.
Is this true? "Every person ovulates on day 14 and all other days are infertile."
Day 14 belongs to a simplified example. Timing varies, sperm can survive for several days, and the fertile phase extends around ovulation.