The endometrium undergoes cyclical changes during the menstrual cycle. What is the main purpose of these changes?

The main purpose of the cyclical changes in the endometrium during the menstrual cycle is to prepare the uterus for the possible implantation of a fertilised egg. The thickened lining provides a rich blood supply and nutrients for the growing embryo. If fertilisation does not occur, the endometrial lining is shed during menstruation, and a new cycle begins.