With medical science, people were studying human development instead of guessing when a soul entered the body, and concluded "yeah, this is a human upon conception."
Yeah... that is bollocks.
Anti-abortion movements date to around the turn of the 19th century, and had precious little to do with the science of when babies could think and feel. It's not that they weren't wrong to realise that the old idea of "quickening" wasn't what the ancients thought, but they also still had no more idea than Aristotle had. At best it was a fig-leaf for the real reasons to ban abortion, pretty much all of which were social factors, such as:
1) Abortions were dangerous procedures whether by abortifacient or quasi-surgical means, with a significant rate of severe illness and death
2) Medical doctors were growing in power and influence; as abortions were generally carried out by non-medics, it was a way of cementing their control over all things health-related
3) Maintaining social control of women
4) Political fear of declining birth rates (thus lower populations and less ability to wage war)
Amusingly, of course, the Catholic church was well behind when it abruptly declared souls to appear at conception, which it did in the mid-late 19th century. The Vatican magicked that line out of approximately nowhere, based on very little except jumping on the social bandwagon already in process.
It was a progressive policy founded in the discoveries of modern medicine.
No.
Firstly, as above. In many ways (e.g. point 3 above) anti-abortion policies were anti-progressive.
Secondly, "progressive" risks being misleading. Certainly, it's got little to do with what anyone would understand as the progressive movement, as that only definably emerged when abortion was all but fully banned anyway. It is fair to say social and scientific progress contributed to arguments around abortion, but that's little more than a pale truism of noting that change happens.