What exactly is a birth defect? I need to wear glasses, isn't that a defect? Wouldn't that mean most of us have a defect? Or is there some level of severity or rarity you have to pass for it to be considered a defect?

You need to have the defect at birth.

A birth defect can be any disability that is present at birth. Your eyes were probably fine at birth (unless you have a stigmatism) and just deteriorated at some point since then. As for disabilities or functional incapacities (if you prefer more syllables), most people do have one or more during their life (and the chances of having one increase with age). To lack any disabilities, while we may think of it as normal, is not the norm.

If you have a genetic defect passed down to you and it doesn't become a problem until later in life, like breast cancer, then I would think it was a birth defect also. One day , with the advent of genetic engineering, bad genes will be removed that could harm us, in the future before we are born.

If your Mom is shot in teh stomach and it blows off your itty bitty hand, is that a birth defect?