20 week ultrasound

Ok I am 16w6d pregnant and I went for a ultrasound last week and the dr told me I should empty my bladder before hand so he can see the baby better. I have a 20 week ultrasound coming up sept 1 and they told me to drink 3 glasses of water before I go a hour and a half before I go and don't pee. Is this nesasary? Cause the other dr told me not to have a full bladder for him to see it?