What medical evidence is he talking about? Babies born before 20 weeks have a survival rate of 0%, never mind 12 weeks! At that point their organs are just starting to function; there are no pain pathways developed and no brain activity. Yes, it's alive, but it's alive at 2,4,6,8,10 weeks too, so he can't be basing his argument for 12 weeks on that fact. The 24 week limit makes some amount of sense because it is around that time that the baby develops the ability to survive independently of the mother (albeit with intensive medical intervention). Twelve weeks seems to be a figure that's been plucked out of the air.