During days 1 through 10 of the menstrual cycle, the levels of estrogen in the body gradually increase. This increase in estrogen levels corresponds to changes in the thickness of the uterine lining, also known as the endometrium.

Estrogen is responsible for stimulating the growth and proliferation of the endometrial cells, causing the lining of the uterus to thicken in preparation for a potential pregnancy. As estrogen levels rise, the endometrium becomes thicker and more vascularized, with an increased number of blood vessels to support potential implantation of a fertilized egg.

This process is known as the proliferative phase of the menstrual cycle and typically occurs from days 5-14. By the end of this phase, the endometrium can be up to 5-6mm thick.

If fertilization does not occur, estrogen levels will eventually decrease, leading to the shedding of the endometrial lining and the start of a new menstrual cycle. However, if fertilization does occur, the thickened endometrium provides a suitable environment for implantation and the subsequent development of the embryo.

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