In the transformation phase, also known as the luteal phase, the empty follicle in the ovary is transformed into the corpus luteum.
The corpus luteum produces the hormone progesterone, which prevents the maturation of further oocytes and supports the build-up of the endometrium, such as the supply with nutrients and its thickening.
If the egg cell is successfully fertilised, implantation into the lining of the uterus would take place. If, however, the egg cell has been nudged through the fallopian tube without having met a sperm, the corpus luteum breaks down and the production of progesterone is reduced. The same applies to estrogen. Menstruation then starts and the cycle begins all over again. On to the next one.