My daughter was born on may 28th and the dr told me she had a very thin umbilical cord. Can any one tell me what causes that or any concerns I should have if I where to become pregnant again? Thanks in advance

Unless your daughter had health problems caused by the cord, I don't think it's anything to worry about. It's very likely right up there with "Oh, what big feet that baby has", or "That's a very bald baby".

Even if it did cause problems, it's almost certainly a one-off, unique to that pregnancy, and unlikely to happen again.