What about when the fetus has a medical condition that will cause him to die at birth?

Just because we know that a person is going to die in the future does not give us the right to kill that person. The fact that a terminally ill five-year-old is going to die soon and is a burden on his parents does not give his parents the right to slit his throat.¹ Therefore, the question is not whether the fetus will ever die (he will) or even when he will die. It is whether or not the fetus is a person. If he is, then it is not morally permissible to dismember him just because he is terminally ill. If the fetus is not a person, then it should be permissible to kill him regardless of whether or not he is terminally ill.