There are a bunch of different ways to define the beginning of life.
Some folks prefer sentience as the starting point. Without sentience (or your consciousness), there is no suffering. I think people measure sentience by the existence of certain brain features.
Clo talked about independent viability, which, as you point out, makes for a moving target.
My view is different from yous and clo's. I want to permit private decisions, but only well within the zone of sentience. But beyond that, instead of thinking about biology, I think in terms of how long it takes a responsible person to make a decision. Pregnancy tests mean the countdown clock for making a choice can begin very early. I think 13 weeks/3 months is long enough to decide what you want to do. So the first trimester is my line in the sand for personal choice.
Extensions for various special reasons, such as life of the mother, suffering of the foetus etc.
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