What Preparation Is Needed?
After you make your appointment, your doctor will give you an instruction sheet telling you what you need to do. Generally, women who are less than 14 weeks pregnant will be asked to fill their bladder to capacity. Sound waves travel better through liquid, so a full bladder improves the quality of an ultrasound during early pregnancy. As a woman's pregnancy progresses, a full bladder is not as essential because the uterus and fetus are so large. But even then, some physicians ask their patients to come with a full bladder because transmission of the sound waves is better with fluid in the bladder.
Vaginal probes are used early in pregnancy (12 weeks and earlier) because sound waves don't pass through bone. The fetus is deep in the mother's pelvis in early pregnancy.