FSH and the start of the cycle

1. At the end of a cycle the concentrations of oestrogen and progesterone in the blood are low.
2. With little inhibition acting on it, the pituitary gland secretes FSH into the blood.
3. FSH is carried in the blood to the ovaries, which are its target organs.
4. In an ovary, FSH makes a follicle containing an immature egg grow and the egg inside it mature.
5. FSH also stimulates the ovary to secrete oestrogen, so the oestrogen concentration starts to rise from about day $4$.
6. The rise in oestrogen is therefore a direct consequence of the FSH released at the start of the cycle.

Oestrogen inhibits FSH

1. Oestrogen from the ovary travels in the blood to the uterus, where it makes the lining grow thick and rich in blood vessels.
2. Oestrogen also travels to the pituitary gland, where it inhibits the release of FSH.
3. Less FSH means no further follicles are stimulated, so usually only one egg completes its development.
4. At the same time oestrogen stimulates the pituitary to secrete LH.
5. Because a rise in oestrogen causes a further rise in LH, this part of the cycle is an example of positive feedback.
6. The LH concentration therefore climbs steeply and reaches a sharp peak at about day $14$.

LH triggers ovulation

1. The sharp peak in LH acts on the mature follicle in the ovary.
2. The follicle bursts and releases the egg into the oviduct, which is ovulation, at about day $14$.
3. The empty follicle stays behind in the ovary and develops into a yellow structure of dividing cells.
4. That structure secretes progesterone, so the progesterone concentration rises from day $14$ onwards.
5. Oestrogen output drops slightly just after ovulation, then rises again as the empty follicle develops.

Progesterone maintains the lining

1. Progesterone acts on the uterus and maintains the thick lining, keeping it ready to receive a fertilised egg.
2. Progesterone also acts on the pituitary gland, where it inhibits the release of both FSH and LH.
3. With FSH inhibited, no new follicle matures, so a second ovulation cannot happen in the same cycle.
4. If the egg is fertilised and implants in the lining, progesterone stays high and the lining is kept in place.
5. If the egg is not fertilised, the structure in the ovary breaks down and progesterone and oestrogen concentrations fall.
6. Once progesterone is low, the lining is no longer maintained and it breaks down as menstruation.
7. Low progesterone and oestrogen also stop inhibiting the pituitary, so FSH rises again and the next cycle begins.