Why does the uterus lining start to grow thick and spongy as an egg cell develops in an ovary?

The uterus lining thickens and becomes spongy due to the influence of hormones, particularly estrogen, released by the developing follicles in the ovaries. As an egg cell matures, estrogen levels rise, stimulating the endometrium to prepare for a potential implantation of a fertilized egg. This thickening provides a nutrient-rich environment that supports embryonic development if fertilization occurs. If the egg is not fertilized, hormone levels drop, leading to the shedding of the uterine lining during menstruation.