16 week ultrasound

I’m about a week away from my 16 week appointment. My doctor has expressed that she will most likely do an ultrasound to check for any abnormalities and also to see if we can find out gender. I’ve heard that sometimes baby doesn’t want to cooperate, they’re too still or won’t move their legs - any advice to help make sure my baby is active enough that they can see what genitalia they’ve got? Hubby is really excited that we may find out gender. But I’d hate for him to be let down, so I want to see if there is anything I can do to help make sure we get the best ultrasound possible.