What Is The Purpose Of The Thickened Lining In The Uterus?

The inner lining of the uterus which is called the "endometrium,"
becomes thicker as the ovary is preparing to release an egg. This
thickening is in preparation for receiving the egg and allowing a
surface for the egg to couple with a man's sperm. If this fertilization
takes place, the uterus' inner lining continues to become thicker to
house the new embryo, and eventually to form a "placenta" (organ to transfer nourishment from the mother) for the developing fetus.