The uterus is the organ where the embryo implants and develops during pregnancy. It has two main layers:  

- The myometrium—a muscular layer that helps support pregnancy and contracts during labor.  

- The endometrium—a superficial lining that undergoes changes throughout the menstrual cycle in response to ovarian hormones (estrogen and progesterone).  

Each cycle, the endometrium thickens to prepare a suitable environment for an embryo. If implantation does not occur, the endometrial lining sheds and is expelled, which results in menstruation.