The whole pick an X day thing is really the wrong approach to take on this, because it easily falls into day-1 regress.
Meh, not really. We can look at any number of relevant milestones in the human development process.
We know that early enough in the process, a fetus is definitely not capable of feelings or consciousness, and therefore has no more intrinsic value than a tumor. Prior to the 7th week, the fetus shows no signs of even rudimentary consciousness. It is not moving or reacting to stimuli. Consequently, any abortion law that does not allow for at least
pre-7th-week abortions at its most conservative is based on some kind of mystical belief about souls.
From that point until roughly 26-30 weeks, all fetal behavior is behavior that can potentially be produced by a brainstem, independent of the cerebral cortex - ie, reflexive behaviors. Everything that we associate with consciousness occurs in the cerebral cortex. You start to get neurons developing in the area that will be the cerebral cortex as early as the 8th week, but you don't actually see activity until roughly week 26. You also don't see anything resembling a pain response until week 26.
The cerebral cortex never stops developing, even throughout all of life, but what we're usually interested in early in life is cortical folding. Cortical folding is associated with all sorts of cognitive abilities; interrupted or abnormal folding is associated with mental retardation, schizophrenia, and all sorts of nasty stuff. The folding process also begins at week 26, and keeps going after birth.
You'll notice that I've mentioned week 26 three times in this post. Well, there's also something called the limit of viability - what's the point in gestation where you can induce a pregnancy and have a 50% chance or better that the premature baby will survive on its own? That's week 24. However, even at week 24, the chances of the baby having severe lifelong disabilities is high. Most of those disabilities have a significantly lower chance of occurring if you wait until week 26.
After week 26, you start to get a person, but not immediately. That's when things get fuzzy. Even for the first few weeks post-birth, the baby is only showing very simple behaviors - pain response, hunger response, thirst response - but you don't even get emotional behaviors right away.
So prior to week 26, you have an entity with:
- Reflexive behaviors, but no behaviors resembling conscious human behaviors
- No activity in the areas of the brain associated with human consciousness
- No pain response
- No cortical folding
- Very poor (or zero) chance of survival outside of the womb
Prior to week 7, you have an entity with:
- Nothing resembling a brain at all
There are the limits to your regress. At the most conservative, we have a week 7 regress; but realistically our regress stops at week 26.
It strikes me that there's our logical cutoff point. If we approach the question from the perspective of fetal humanness, week 26 is a solid minimum marker. If we approach the question from the perspective of mother independence, week 26 is where we can realistically separate the fetus from the mother without extremely high risk to either one.