You don't pee from your uterus! You pee from your urethra.

Another thing: You have to go through the cervix in order to reach the uterus. Just how do you think you could shove something through your cervix and into your uterus? The cervical opening is only about as big as the lead in a pencil, and you have no way of dilating the cervix far enough to allow you to shove anything through it.