They certainly knew how conception took place. They knew that a "life" was growing. That was not the issue. The issue was the theory or concept of ensoulment, which has no scientific basis. When Pius IX issued his determination that a fetus had a soul at conception, it still had no scientific basis, nor is their any evidence he had any more biological knowledge than did Augustine, whose determination of no ensoulment was based on the unformed and non-animated body of the fetus, which are still facts today.
I've been studying this issue for some time now. The science is clear, biogenesis is the law of the land, new human life begins at conception
You should give Augustine some credit. He knew that the life growing from a "seed" was indeed a human, but so unformed as to make it unready for the soul. No amount of biogenesis is sufficient to counter the point made by Augustine. The early fetus has no central nervous system, no arms, legs, sexual distinction, mind, nor any of the physical attributes that distinuish a human being from any other early stage animal. Augustine's determinations of ensoulment and Pius' later countermanding of that are both simply philosophical calls, not based on any science.
This is purely an article of faith. Both sides of the issue have credibility...until the fetus is in its later term. Then, I believe, common sense and the Constitution come into play.
Nobody gets a pass here at FR
You got that right!
I posted a thread on that several years ago about this lucunae of Catholic doctrine (before it was dumped in the 19th century), in a larger essay by Easterbrook on fetuses and sentience, which was not well received. I reposted it years later, and someone observed that most of the participants from back then had been banned. And I am still here! Maybe miracles are not impossible. Anything is possible.