I had the exact same thing happen to me. When I had my 18 week scan done they couldn't see Declan's brain properly and I think his spine. The old guy I had tried to imply it was my fault due t me being a little bigger (although he didn't say it in those words) but I had another scan done somewhere else and everything was fine, which I am sure will be in your case too. Sometimes it is the u/s machine they are using aswell as bubs not co-operating properly.

Good luck.