You need to arrive with a full bladder for both early scans and gender scans between 16-20 weeks gestation. After 20 weeks it is not necessary but can be useful if your baby is in a difficult position. Instead we would kindly ask if you avoid going to the toilet 15-20 mins before your scan. To obtain a full bladder, you need drink approximately 2 cups of water about 40 minutes before your appointment. If you become uncomfortable please go the toilet. Your bladder will fill fairly quickly again if you are well hydrated.