Final answer:
The endometrium is the mucous membrane lining inside the uterus that thickens during the menstrual cycle to prepare for the implantation of an embryo, shed if no fertilization occurs, and nourishes the embryo if pregnancy happens.
Step-by-step explanation:
The endometrium is the mucous membrane lining the uterus, which thickens during the menstrual cycle in preparation for possible implantation of an embryo. It consists of two layers: the stratum basalis and the stratum functionalis, with the latter undergoing thickening in response to hormones. In case a fertilized egg does not implant, the stratum functionalis is shed during menstruation. The thickened endometrium provides nourishment to an embryo if implantation occurs, and also develops into the maternal portion of the placenta, which facilitates the exchange of nutrients and waste between mother and fetus.
The uterus itself is about the size of a woman's fist, rich in blood vessels and mucus glands, and has a muscular wall made of smooth muscle aiding in contractions during labor. The cervix is part of the uterus that helps form the birth canal. During the luteal phase of the menstrual cycle, spiral arteries supply the endometrium with blood, further supporting its growth and potential to support an embryo.