MY DD was also born with a very deep one that they couldn't see the bottom of. We were also sent for an ultrasound of it, as when they can't see the bottom they are afraid it could been tethered to the spinal cord, which can impede walking. Her's was fine, it is very very rare, and the paed also told me it is not the same as a neural tube defect in that a lack of folate causes it, it is quite common and there is nothing anyone can do to prevent it (I was paranoid missing my folate sometimes when pregnant caused it).

The ultrasound took around 5 mins, and they gave me the answer on the spot. I think we did it when she was 3 or 4 weeks old.

You can still see it on her now, it's a little weird but plenty of people have changed her and nobody has noticed unless I point it out. It is so small it's not like you can clean it, so other than washing her skin like any part of her body, there is nothing special I do. Never had any issues with any infections there or anything. She is 4 now.

Hope that help