I don't know, Im not really interested in the subject of abortion. I suppose when the fetus can react to a stimulus like heart beat decreasing when the mother talks, moving around when taping on the stomach, something along those lines. It would be difficult to determine in each case which is why I think they just go with general terms like # of weeks or heart starts beating. It's also an uncomfortable topic to discuss for most people, thinking about the exact line when you can terminate a pregnancy before the fetus becomes conscious.
I'm just wondering. Drawing a line between "sentient" and "not sentient" to me would vary from fetus to fetus with regard to the exact time that occurs.
Without a test, the best way I can see is to set an absolute minimum time of development where sentience CAN happen, and make abortion legal before that point including an additional margin for error.
In most pregnancies, the date of conception is a guess. All developmental timelines, including delivery date, are based on that initial date. So, there should also be and additional margin for error added based on the likelihood the conception date was close.
Basically, conception date - that margin of error + date of sentience - that error margin = last day to abort.
Those margins of error are what concern me most. If there's not a reasonable degree of accuracy in the estimates, you're running a risk of violating your "rule" that abortions must be before sentience. If you miss that estimate by a few days, you just murdered a sentient human being.
It's a complicated and risky process when we try to give ourselves the greatest leeway possible to abort.