From AP:

Authorities said Montgomery, 36, confessed to strangling Bobbie Jo Stinnett of Skidmore, Mo., on Thursday, cutting out the fetus and taking the baby back to Kansas.

[Bold mine.]

The logic? There’s no “baby” when it is not clear if “it” is wanted. Once it was established that the kidnapper saw the “fetus” as a “baby” she wanted, “baby” was the safe word. If you’re wanted, you’re alive. If you’re not, or their is still time to make the choice, you’re in existential limbo.