Why you need to have a sonnomammogram 8-10 days from the start of your menstruation?

Measure endometrium: Most ob/gyns prefer to order a pelvic ultrasound shortly after a menstrual period so that the lining of the uterus also known as the endometrium has just shed and is at its thinnest.

Ultrasound.: I think you are talking about a pelvic ultrasound. A pelvic ultrasound is done at that time because the lining of your uterus is at the thinnest. It allows the radiologist to measure the correct thickness of your endometrial lining. As you menstrual cycle continues the lining of your uterus increases in size until you have your period.