3.9 understand the roles of oestrogen and progesterone in the menstrual cycle

There are four stages in the menstrual cycle:
Stage 1: Day 1 - Start of menstrual cycle. Uterus lining (endometrium) breaks down for around 4 days.
Stage 2: Day 4~14 - Uterus lining builds up again into thick spongy layer full of blood vessels, ready to receive a fertilised egg.
Stage 3: Day 14 - Ovulation; when a matured egg is released from ovary
Stage 4: Day 28 - Uterus wall is maintained for another 14 days until day 28. If no fertilised egg lands on uterus wall by day 28, cycle repeats

Oestrogen:
- produced in ovaries
- causes lining in uterus (endometrium) to grow
- inhibits the release of LH (Luteinising Hormone) which causes ovulation

Progesterone:
- produced in ovaries at around day 14
Progesterone has two main functions:
1. maintaining uterus lining (endometrium) during the second half of the cycle. When the level of progesterone decreases, the lining breaks down.
2. Inhibits the release of LH and FSH. Progesterone also causes the pituitary gland to stop secreting hormones FSH and LH so a new cycle is never started. For this reason most birth control pills contain progesterone

FSH (Follicle-stimulating Hormone):
- produced in pituitary gland
- causes egg to mature
- stimulates ovaries to produce oestrogen
- estrogen inhibits the release of FSH

LH (Luteinising Hormone):
- produced by pituitary gland
- stimulates ovulation