There are several layers or strata (sing: stratum) in the structure of the uterus. The two major layers are the myometrium and the endometrium. The myometrium is the underlying muscular layer, and the endometrium includes the layer that provides new growth as well as the layer that is shed during the menstrual_phase of the ovarian_cycle.
The endometrium is divided into two strata, the stratum_functionalis and the stratum_basalis. Each month, new layers of cells and uterine (a/k/a endometrial) glands grow from the stratum basalis to become the stratum functionalis. The increase in hormone levels during the early part of the monthly cycle support this growth. If no egg is fertilized, the hormone support drops (as the corpus luteum shuts down) and the cells begin to fail. Menstruation then begins.