The endometrium, which consists of two layers called the functionalis and basalis, goes through changes during the menstrual cycle. The changes that occur are needed for successful implantation of an embryo. The proliferative phase of the menstrual cycle is primarily governed by estrogen and is responsible for the thickening of the endometrium. Progesterone primarily controls the last half of the menstrual cycle and causes changes which allows for embryo implantation.