The endometrium is the tissue that lines the uterine cavity, in which the embryo must implant. This is an active tissue (it is not a mere pad where the embryo implants) and its behaviour is decisive for this implantation to occur.
The endometrium being such an important tissue for the reproductive system, it must be continually renewed. In non-pregnancy conditions, the endometrium sheds, usually every month, which is known as menstruation.
During menstruation, the one that was formed in the previous cycle is discarded, so that just after it, it is thin, and must grow, due to cell proliferation, to generate a new endometrium in which an embryo can implant. This proliferation is controlled by the hormonal system of women, mainly by estrogens, which are secreted mostly in the ovary of women, during the growth of the follicles, which are directed towards ovulation.
Endometrial hyperplasia is an uncontrolled growth of the endometrium, without implying malignancy in its cells. Most of the time (although not always) it occurs due to excessive hormonal stimulation, for example, prolonged estrogen secretion or administration. Endometrial hyperplasia, therefore, is nothing more than the situation in which the endometrium produces, as always in the first part of the cycle, before ovulation, cell proliferation and growth of the endometrial thickness, but in an excessive and disorderly way. This makes the endometrium tend to be thick, that the achievement of pregnancy is complicated in this cycle (the endometrium is not correctly ordered and balanced cellularly and molecularly speaking) and sometimes, if prolonged, can cause bleeding due to spontaneous desquamation as this endometrium is too thick to be able to remain stable in the uterus. This type of hyperplasia is called simple hyperplasia is what is typically seen in the fertility centres and is easily controlled with hormonal treatment.
However, if the situation continues without diagnosis and treatment, the cellular disorder is evident, which is why we speak of complex hyperplasia, in principle without atypia (prolonged and specific medical treatment). The worsening of the hyperplasia will make the cells no longer normal, so they begin to look like tumour cells. This is complex hyperplasia with atypia, which is the pre-stage of endometrial cancer, and which must be treated quickly by surgery. If the situation continues and the endometrium worsens, endometrial cancer is declared, the treatment of which is much more complex.