I'm not quite sure on what you are asking. Those are tricky nubs in the pics you have posted--with some of them we aren't seeing the whole nub as baby is just a bit off center from capturing it completely. In others the back is a bit curved. I will admit that a couple I would have guessed girl for sure on but I don't know the gestation of all of them either and I don't knowingly guess before 12 weeks. Nubs start out flat for the most part and gradually angle up over the course of the 11th week and sometimes early into the 12 week so that could be the case in some images as well.
Really, the best way to learn is to keep guessing and asking questions :)

