More from my Medical Self-Defense paper; see the original for footnotes, should you want it. Thanks also for your comments: Among other things, they've persuaded me that I need to explain in more detail why I use the self-defense defense as an analogy instead of the necessity defense -- I think I have a good justification for it, but it sounds like I'll need to make it explicitly, and I'll do that on-blog and likely in the article as well. In the meantime, a brief discussion of imminence, to be followed shortly by a discussion of the constitutional law and self-defense:

Lethal self-defense is generally allowed only in response to imminent threats of harm, usually measured in minutes; medical self-defense would often be used to prevent a death that's likely in months. But for medical self-defense, it makes sense to treat imminence as simply requiring a present medical condition that seriously threatens life in the relatively near future -- that is to say, as an application of a necessity requirement -- not as requiring that death be likely within the hour.

The imminence requirement in lethal self-defense is aimed at serving several functions. First, imminence is a rough proxy for necessity of lethal response: Lack of imminence is correlated with the presence of alternatives (escape, calling the police, and the like) and the possibility that the threatener's anger will cool. Second, the imminence requirement diminishes erroneous claims of necessity, since judgments about long-term threats tend to be less accurate than judgments about short-term threats. Third, the risk of false claims of self-defense would be especially high if "he had told me some time before that he wanted to kill me" were justification enough for killing. Fourth, insisting on tools that help show necessity and help weed out false claims is especially important because unnecessary self-defense causes unnecessary death.

For medical self-defense, all these functions would best be served by seeing imminence as simply requiring a present medical threat. The best proxies for necessity are the present medical threat (your kidneys are actually sick) and the lack of a satisfactory permitted therapy. You can't flee kidney disease that can be cured only through a transplant, or call the police to protect you. Present medical threats of future harm are generally more reliably diagnosable than human threats. There is little risk of insincere claims of danger, especially since the diagnosis is made by a trained and objective medical expert. And even if an error happens, it will likely endanger others far less than erroneous lethal self-defense would.

These reasons help explain why the law doesn't distinguish a woman who gets a post-viability abortion to protect her life against immediate danger (her pregnancy was threatening likely immediate death) from a woman who gets a post-viability abortion to protect her life against real but not immediate danger (her pregnancy was threatening likely death in a month). So long as the dangerous medical condition currently exists, the woman may defend herself against the risk right away, especially since waiting may increase the danger. The same should apply to other forms of medical self-defense.