Ultrasound!?

I’m very curious to know what makes the ultrasounds come out with better quality around the 12-13th mark? I’ve just noticed with peoples on here some look extremely clear and others are hard to see. I was just wondering if there was a reason to this as maybe baby was too far back (which we can’t control). Or make sure you drink plenty of water before the scan? I have my scan next week and I want to use the photos for an announcement so I’m hoping to get clear ones. TIA!