Generally in the early stages of pregnancy (usually the first trimester) the uterus is not large enough to exit the pelvis, so it is necessary to hold enough urine to help see the uterus as well as the fallopian tubes and ovaries using a full bladder. In the middle and late stages of pregnancy, it is usually no longer necessary to hold your urine, unless there are special circumstances, such as a low placenta and unexplained vaginal bleeding, that require the use of a full bladder to see the tissue and blood flow in the lower part of the uterus.