Your rationale only works if God created Adam and Eve as imperfect beings.
Not really. People on this thread have claimed that birth defects are the punishment exacted from God as a result of original sin.
If God doled out the punishment for original sin which was to be suffered over the generations, you then have to acknowledge that God altered human DNA in order for some to produce a certain % of defective offspring.
If you don't think birth defects are a consequence of original sin, but do believe that God created man, then it follows rationally that God simply included chromosomal anomolies in some humans from the start.
Or, you could believe that some other entity, like Satan had a hand in it, although there is no biblical reference to support that, is there? (The whole original sin thing is mostly a Paul project... does anyone else mention it even?)
But if you believe that birth defects are simply the result of nature screw-ups and God has nothing to do with any of it, then I hate to break it to you, but you're arguing for evolution and natural selection.