Wow, it has a name!
Caden has one also, and the pead just checks that it has a "bottom" and is sealed. Apparently it is the last thing to "zip" up once the spinal cord has been sealed, and is quite common.

I thought they could check for spina bifida in utero?

I also thought it was genetic, as it can be passed down in families. I thought I would take extra folate next pregnancy just in case, but am not too worried as my doc isn't worried.

Good luck!