How useful is the concept of the 'harm threshold' in reproductive ethics and law?

In his book Reasons and Persons, Derek Parfit suggests that people are not harmed by being conceived with a disease or disability if they could not have existed without suffering that particular condition. He nevertheless contends that entities can be harmed if the suffering they experience is sufficiently severe. By implication, there is a threshold which divides harmful from non-harmful conceptions. The assumption that such a threshold exists has come to play a part in UK policy making. I argue that Parfit's distinction between harmful and non-harmful conceptions is untenable. Drawing on Kant's refutation of the ontological argument for God's existence, I suggest that the act of creation cannot be identical with the act of harming-nor indeed of benefiting-however great the offspring's suffering may be. I suggest that Parfit is right that bringing children into existence does not usually harm them, but I argue that this must be applied to all conceptions, since Parfit cannot show how the harm threshold can be operationalised. If we think certain conceptions are unethical or should be illegal, this must be on other grounds than that the child is harmed by them. I show that a Millian approach in this context fails to exemplify the empirical and epistemological advantages which are commonly associated with it, and that harm-based legislation would need to be based on broader harm considerations than those relating to the child who is conceived.