Which line is the nub??

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My husband and I aren’t finding out the sex of the baby but after our 19 week ultrasound today I am dying to know lol. So I’m trying to go behind his make and make a prediction 😂 Does anyone know which line is the nub? The top part or bottom part-they both look like they could be. Maybe it’s neither and this isn’t a good nub theory pic? 🤷🏻‍♀️ Any help would be appreciated!