Uterus. Coloured 3-D ultrasound scan of a healthy human uterus (centre) prior to menstruation. The lining of the uterus (endometrium, dark brown surrounding the uterine cavity) is at its thickest around the time of ovulation, when it is ready to receive a fertilized egg. The lining is rich in glands, connective tissue and blood vessels. In the absence of a fertilized egg, the endometrium is shed and passes out of the vagina during the monthly menstruation. Ultrasound scanning is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure.