Guess the NUB!
Yes, makes perfect sense. Although everything I've looked at shows a nub side profile.
You'll have to google it because it's kinda hard to explain but basically that line or "nub" protruding from where the baby's butt is can predict if the baby will be a boy or a girl. If it's angled then it's a boy and straight is a girl. Obviously it's more complex than that and can be close but it seems to work for a lot of people.
Here is a link!