Spicy, I think nub theory is at it's worst on still photos, especially if there is only one pic. I think ELP's first nub scan photos are a prime example of that. BUT ... as she mentioned when she posted them, she SAW a boy nub during the scan. So... and this probably isn't super encouraging, I do think if you're SURE you saw, repeatedly, a flat looking nub during your scan, then you're probably making a very educated guess. It's easy for me to hold out hope since I didn't see it but you know what you saw.
How old is your DD1 again? Mine are 3.5 years apart, this one and DD2 will be 2 years 10 months. I was actually glad this morning that we didn't wait (we had originally planned to try to get pg this fall, instead of back in summer) because IF we have a 4th I can still pop it out before I'm 36.