How do they tell by this!?

Lacey • Married since 2016❤️•Mommy of 7 👧🏻👧🏼👧🏻👧🏻👧🏼👶🏻👶🏻•
How can people tell by an ultrasound picture as to what they're having? Is it the baby's position or what? I'm on my 4th kid and I had no idea about this! Lol