Murder of Unborn Children as Capital Crime?

Flores v. State (Tex. App. Jan. 24, 2007), upholds a life sentence for a father's capital murder of his unborn twins. (Flores had admitted that "in the seven days prior to her delivery [he] stepped on her abdomen on two different occasions.")

Flores argued that the statute "violates the Establishment Clause of the United States Constitution because defining life as beginning 'at conception is religious, not medical or legal'"; that was properly rejected under Harris v. McRae. But what if he had been sentenced to death? Would that have been precluded by Coker v. Georgia -- which held that the death penalty for rape was unconstitutional, and which many have read as generally foreclosing the death penalty for crimes short of murder (but which the Lousiana Supreme Court has interpreted, controversially, as allowing the death penalty for rape of children)? Would it matter how old the fetuses were, and were would the constitutional line be drawn on that?