Chris Heard, a rather strong anti-abortionist, sums it up like this:
“Let me be completely clear and honest: I despise abortion. I think that a biblically-informed valuation of human life leads one in that direction. But I also object to bad exegesis. There is no biblical proof-text against abortion. Deuteronomy 30:19 (“choose life”) has nothing to do with abortion; it has to do with being party to God’s covenant with Israel. Psalm 139:13-18 is less relevant to the issue than most people think; a careful reading of that psalm reveals that the “mother” in whose “womb” the psalmist was known by God is Mother Earth (notice the parallelism between “my mother’s womb” and “the depths of the earth” in the inclusio of vv. 13-15). Exodus 21 is very difficult, but it certainly does not speak directly to abortion; at most, it relates to an accidentally induced miscarriage, though it may refer to a premature birth. That interpretive decision is crucial, and I’m not sure how to resolve it. As far as I can tell, the only biblical passage that I know of that directly mentions a practice like we would think of as abortion curses a man who did not practice it on the fetal Jeremiah (Jeremiah 20:14-18).”
Indeed, in the Jeremiah passage the prophet curses the day he was born and laments the fact that he was not aborted — hardly of help to the anti-abortionist cause.
Heard does believe that a biblical case can be made for anti-abortion, but that it would have to be a “cumulative theological case” rather than a direct case based on proof texts. I think he’s probably right (for the record, I’m as pro-choice as they come), and history speaks for itself. The Jews and Christians of antiquity were known for despising infanticide and abortion.
In fact, in the book I recently reviewed, Mohammaed & Charlemagne, Emmet Scott revisits the historical claim that Constantine adopted Christianity, at least in part, to halt the population decline in the Roman empire. As early as the end of the first century, people like Tacitus and Pliny the Younger complained about the problem of childlessness and the common view of children as a burden; baby girls were especially unwanted and discarded. The only groups in the empire that were increasing by normal demographic process were the Christians and the Jews (in no small part because they extended the sanctity of life to children, infants, and the unborn), and Constantine may have been trying to capitalize on this.
Anti-abortion, in other words, is not biblical in the way that homophobia, post-tribulation eschatology, and (NT) pacifism are. It’s more biblical in the way that anti-racism or (NT) separation-of-church and state is. A convincing case can be made for it by building on related biblical ideas, but the platform doesn’t span the bible.