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

Oestrogen causes the lining of the uterus to thicken and grow. It also stimulates the release of an ovum (well, it stimulates the release of LH which stimulates the release of an ovum).

Progesterone maintains the lining of the uterus. The lining breaks down when the level of progesterone falls.