Nub theory

This is my 13 weeks ultrasound. I'm now 17wks. I've been trying to find this "nub" everyone keeps talking about. I can't see it though. Help? I don't find out the sex for another 4wks, and I'm really anxious. I want a boy, but my boyfriend wants a little girl. However We're happy no matter what, as long as it's healthy and happy. 

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