Killing an innocent human being is prima facie wrong. A fetus is an innocent human being.
Abortion is the killing of a fetus.
Therefore, abortion is prima facie wrong. One of the argument's key terms is ambiguous. The premises are all true only if there is equivocation within the argument. The argument is valid only if there is no equivocation within the argument. There is no non-arbitrary dividing line between a zygote and the person it becomes.