It seems like most disorders that show up at birth are because of a totally random simple mistake while the fetus is forming. I know it could be hereditary too, but my opinion is that, at least with sacral agenesis, it's kind of a random occurrence.
One small mistake in DNA formation and development, and the child is born with a condition that changes his or her life. And I think the rate of death with infants who have sacral agenesis is pretty high, right?
If I ever decide to have kids I will definitely take my folic acid to prevent sacral agenesis during pregnancy.