Why and how does a full bladder help with ultrasound?

A "full bladder" will create a "window" of unobstructed passage of ultrasound to outline the uterus and ovarian anatomy, which makes a more accurate determination possible, since the bladder walls are separated, decreasing the density of tissue overlying the female reproductive organs.

Full bladder helps identify it and the surrounding structures better as ultrasound is transmitted better thru clear fluid (urine)