Just now, I am also reminded of People v. Kurr (2002), a Michigan case that determined deadly force could be used, in defense of others, for a fetus. In Michigan. Specifically in light of relevant statutes passed by the legislature. So again, it’s not that this question just hasn’t come up—it has—it’s that whether “defense of a fetus” might be allowed (whether the argument could even be put forward in front of a jury) as a defense to murder or not will depend, first of all, on the particulars of the law in that jurisdiction. If the legislature has enacted statutes that give rise to notions of fetal personhood, then quite possibly defense of a fetus might be allowed as a defense to murder. But if it hasn’t, then common law would likely still apply, in which case a fetus isn’t a person or “human being” as far as the law is concerned.
Which I hope highlights just how difficult these questions can be to answer in the abstract. What might amount to a meritorious defense in one jurisdiction could be deemed frivolous (or at best novel, and ultimately not allowed to even make it in front of a jury) in another.
https://caselaw.findlaw.com/mi-court-of-appeals/1081169.html
And as previously noted, even if a particular defense might theoretically be allowed as a matter of law, the facts might be so at odds with the necessary elements to successfully raise the defense as to make it meaningless. It’s one thing to say “He was coming at me pounding his fist into his palm and saying he was going to beat my kids out of me, so I picked up a kitchen knife and stabbed him.” It’s something else altogether to say “I found out this guy performs abortions, and we all know those are illegal, so I put a pipe bomb beneath the driver’s seat of his car and rigged it to go off just as soon as the car started moving again.”
ETA: Here’s a helpful blurb from Kurr, highlighting how variations in statute (or whether a statute even exists) across jurisdictions—not only in terms of who is a person, but the requirements for self-defense, the definition of murder, etc.—can lead to very different outcomes:
… Moreover, the legislative analysis of the act indicates that, in passing the act, the Legislature was clearly determined to provide criminal penalties for harm caused to nonviable fetuses during assaults or negligent acts against pregnant women. See Senate Fiscal Agency Legislative Analysis, SB 21 and HB 4524, August 4, 1998.
Because the act reflects a public policy to protect even an embryo from unlawful assaultive or negligent conduct,5 we conclude that the defense of others concept does extend to the protection of a nonviable fetus from an assault against the mother. We emphasize, however, that the defense is available solely in the context of an assault against the mother. Indeed, the Legislature has not extended the protection of the criminal laws to embryos existing outside a woman’s body, i.e., frozen embryos stored for future use, and we therefore do not extend the applicability of the defense of others theory to situations involving these embryos.
We acknowledge that in Ogas v. Texas, 655 S.W.2d 322, 325 (Tex.App., 1983), the Texas Court of Appeals rejected the defendant’s assertion that she was entitled to use deadly force to protect her unborn child. However, unlike Michigan, Texas has codified the defense of others theory in its criminal statutes. See Tex Penal Code Ann 9.33. Section 9.33 provides that one may use deadly force against another to protect a third “person,” and “ person” is defined by the penal code as “an individual, corporation, or association.” See Tex Penal Code Ann 1.07(a)(38). Moreover, and significantly, “individual” is defined as a “human being who has been born and is alive.” See Tex Penal Code Ann 1.07(a)(26). Under these circumstances, the court concluded that “an unborn fetus is not included within the statutory definition of ‘person’ and, hence, not included with the provision of Sec. 9.33.” Ogas, supra at 325.
Michigan has not similarly codified the defense of others theory in its criminal statutes, and we are therefore not bound by restrictive statutory definitions. Our Legislature, as noted earlier, has expressed its intent that fetuses and embryos be provided strong protection under the law from assaults against pregnant women, and we believe that our decision today effectuates that intent.
The common law of other states is likewise distinguishable. …