Having a full bladder is not always necessary for a 19-week ultrasound. The requirements may vary depending on the specific instructions given by your healthcare provider or the facility where you are having the ultrasound. In some cases, having a partially filled bladder can help improve image quality by providing better visualization of certain structures. However, advancements in technology have made it possible to obtain clear images even without a full bladder. It is best to follow the instructions provided by your healthcare provider or contact the facility where you will be having the ultrasound to clarify their specific requirements. They will be able to provide you with accurate guidance based on their protocols and equipment capabilities.