Roles of Oestrogen & Progesterone in the Menstrual Cycle

The menstrual cycle is a repeating cycle, usually about 28 days long, that happens in females from puberty onwards. Each cycle prepares the body for a possible pregnancy by building up the lining of the uterus (womb) so that a fertilised egg could implant and grow.

The cycle is controlled by hormones — chemical messengers carried in the blood. For spec 3.9 you need just two of them, both made in the ovaries:

• Oestrogen
• Progesterone

These two hormones work in turn to build up, maintain and then (if there is no pregnancy) break down the uterus lining. Day 1 of the cycle is counted as the first day of bleeding (a period).

Exam tip. For Double Award, focus on oestrogen and progesterone and what they do to the uterus lining and ovulation. You do not need detailed graphs of every hormone — keep it to the clear jobs described below.

After a period, the uterus lining is thin. Oestrogen is the hormone that gets the uterus ready again. It has two key jobs in the cycle:

1. Rebuilding and repairing the uterus lining. Oestrogen causes the lining of the uterus to grow thicker again after menstruation. This rebuilds the soft, blood-rich lining where a fertilised egg could implant.

2. Stimulating LH to trigger ovulation. As oestrogen rises, it stimulates the release of another hormone called LH (luteinising hormone). A surge of LH triggers ovulation — the release of an egg from the ovary. Ovulation happens around day 14, the middle of the cycle.

So you can think of oestrogen as the hormone that builds the lining up and then sets off the release of the egg.

Exam tip. Two marking points for oestrogen are usually: (i) repairs/rebuilds the uterus lining, and (ii) stimulates LH which causes ovulation. Learn both — many students only remember the lining and miss the LH/ovulation point.

After ovulation, the second hormone takes over. Progesterone has one main job in the menstrual cycle:

• Maintaining the thick uterus lining. Progesterone keeps the lining of the uterus thick and ready in case a fertilised egg arrives to implant. While progesterone levels stay high, the lining is kept in place.

What happens next depends on whether the egg is fertilised:

• If a fertilised egg implants → progesterone stays high → the lining is kept to support the early pregnancy.
• If there is no pregnancy → the level of progesterone falls. Without progesterone to maintain it, the lining can no longer be held in place, so it breaks down and is lost through the vagina. This is menstruation (a period), and a new cycle begins.

So the key link to remember is: progesterone high = lining maintained; progesterone falls = lining breaks down (period).

Exam tip. The classic question asks "what happens when progesterone levels fall?" — the answer is the uterus lining breaks down / menstruation occurs. Saying progesterone "controls periods" is too vague for the mark.

It helps to follow the two hormones around one whole cycle. Counting from the first day of a period:

Days (approx.)	What happens	Hormone in charge
1–5	Menstruation — the old uterus lining breaks down and is lost (a period)	progesterone has fallen
6–13	The uterus lining is rebuilt/repaired and grows thicker again	oestrogen rising
~14	Ovulation — an egg is released from the ovary	oestrogen → LH surge
15–28	The thick lining is maintained, ready for a fertilised egg	progesterone high
End of cycle	If no pregnancy, progesterone falls → the lining breaks down again → back to day 1	progesterone falling

Here is the same idea shown as a simple cycle diagram:

Exam tip. Learn the order of events and the approximate days: menstruation (days 1–5), lining rebuilt, ovulation (~day 14), lining maintained. A "describe the cycle" question wants the events in order with the right hormone named for each stage.

Students often muddle the two hormones, so it is worth seeing their jobs side by side:

Hormone	Where it is made	Main role(s) in the cycle
Oestrogen	ovaries	Rebuilds/repairs the uterus lining; stimulates LH, which triggers ovulation (~day 14)
Progesterone	ovaries	Maintains the thick uterus lining; when it falls, the lining breaks down (menstruation)

A simple way to keep them straight:

• Oestrogen = the "build-up" hormone (it grows the lining and sets off the egg's release).
• Progesterone = the "hold-on" hormone (it keeps the lining in place; let go and the period starts).

Exam tip. A frequent 3–4 mark question gives you a statement about each hormone and asks you to match or explain it. The safest marks come from: oestrogen → repairs lining + LH/ovulation; progesterone → maintains lining; falls → menstruation.