A proliferative endometrium is part of the normal menstrual cycle of a fertile woman. The entire process is controlled by the pituitary gland, the ovaries and follicles that are developing in the uterus. The process is at its peak during ovulation and this is when the endometrium is at its growth phase. When pregnancy is not detected, the entire endometrium should be destroyed and shed in the menses. This is what causes the bleeding. Curiously, this is a cycle that finds a mirror in other mammals as well but it is only human and great ape females that the endometrium is shed; in other mammals, it is simply reabsorbed into the body.
The endometrium is proliferative in the follicular phase. This can be better explained with the use of a time line. Assume that day 1 starts with the first day of a period. This is when the levels of luteinizing hormone or LH, follicle stimulating hormone or FSH, estrogen, and progesterone are all equal. During menses the endometrium is being shed. After a period is over on day 5, the levels of FSH from the pituitary gland cause the development and stimulation of follicles or immature ova. The ova then secrete Gondaotropin Releasing Hormones, estrogen, as well as LH. This starts a race for the most dominant follicle to mature into an egg. Estradiol secretions reach their peak around day 14 where ovulation takes place. This is the time when a dominant follicle has emerged and causes the shutdown of the other hormones, effectively destroying the competing follicles. Ovulation can best be termed as the “heat” period when the body is preparing for fertilization of the mature ovum. Between this period and day 28, progesterone takes over as the dominant hormone where it maintains the endometrium in its current size and ensures that a period does not occur. If fertilization is not detected by secretions of Human Chorionic Gonadotrophin or HCG from a fetal cell, progesterone production shuts down and the cycle will start all over again.
The proliferation of the endometrium can sometimes go out of control and grow into the myometrium in adenomyosis, in to the ovaries and fallopian tubes in endometriosis, and ecotopically in any direction. Treating this condition requires that progesterones like norethisterone which is administered from day 1 until day 20. This is done to ensure that the endometrium remains inactive and is broken down with each successive period.