Im not convinced its the nub at all me4. Its not the same color as the rest of the tissue. I think its thigh bone. But I cant be 100% the first shot baby is mid roll which would give that blurred bone movement. In the second one I see your cord but not where it runs down. The second photo I can trace the thigh bone directly through the "nub". So don't give up yet. It could be hiding a boy part but nothing in these to prove either way.