From the Reason article, quoting Blackwood:
Life "begins in contemplation of law as soon as an infant is able to stir in the mother's womb." Under the common law, Blackstone explained, legal penalties for abortion only occurred "if a woman is quick with child, and by a potion, or otherwise, killeth it in her womb."Movement of the baby in the womb can be felt "quickening" as the article said late in pregnancy (around 25 weeks or so). So many have felt that was when the soul entered the body.
However, human fetuses move much earlier, at least at six weeks. This was not known in Blackstone's time, as it could not be felt.
So if we accept fetal movement as the dividing line for legal protection, the age for legal abortion would need to be moved to prior to six weeks.
Here is a video of a seven week fetus moving. This is clearly a living human being:
https://www.youtube.com/watch?v=VjhFNEV6shYHowever, the criteria of movement does not make sense to declare a human being worthy of legal protection. If that were so, it would be ok to kill paralyzed persons.
Laws banning human abortion at earlier stages of development actually started being written towards the latter part of the 19th century- prompted by discoveries in embryology. Science realized that the "quickening" standard was false, and that early embryos and fetuses are living human beings, just like later term fetuses are. This pro-life movement was led by doctors, guided by science.
I do not believe in "ensoulment"... I think it is wrong to kill unborn human beings
because they are living human beings, and therefore deserving of the right not to be killed, despite whatever physical traits they may lack.