The uterus, the organ where pregnancy takes place, has a cavity inside where implantation occurs.  This cavity is covered by a soft tissue called endometrium, the “nest” that the uterus prepares every month for a possible embryo.

Phases of devolopement of the endometrium

During the menstrual cycle the endometrium goes through 3 main phases:  

Menstrual phase, when it is shed in order to grow again. follicular phase or preovulatory, when it grows, and Lluteal phase or postovulatory when it reaches the proper state in which to produce implantation.  These phases of the cycle are easily seen in an ultrasound.