Seen from a philosophical point is that if a child is not brought into the world, it has not benefited of the community and in that sense you can never harm a person by bring it into existence, unless the person's life is so dreadful that nonexistence is preferable.  That life with a disability or chronic illness is predictably worse than non-existence is not plausible for most of the defects for which we test, even Down syndrome, which is the most tested for and common reason for abortion, Where in fact a happy disposition is actually a characteristic trait. Hence, bringing a child into existence cannot count as harming it. (1)  1. Stanford Encyclopedia of philosophy,  , accessed 05/24/2011