From Theories of Biological Development, by Melinda Bonnie Fagan and Jane Maienschein, at the SEP:
Aristotelians followed Aristotle and without much further study of embryos interpreted the process of generation, including human development, as gradual and epigenetic. Traditional Catholicism agreed. St. Augustine and St. Thomas Aquinas both held that hominization, or the coming into being of the human, occurs only gradually. Quickening was thought to occur around 40 days, and to be the point at which the merely animal mix of material fluids was ensouled. Until 1859, when Pope Pius IX decreed that life begins at “conception”, the Church was epigenetic along with the Aristotelians (see Maienschein 2003).
This is somewhat muddled and mostly not right. The Aristotelians did in fact have further study of embryos. Obviously not every Aristotelian did a special study in the way that, say, Albert the Great did (although in a period in which people's daily lives involved much more extensively with animals than they do today, most of them would have seen plenty of embryos from eggs and the like) and obviously they did not have the benefits of later advances; but Aristotle's own conclusions are at least plausible if your means of study are limited, so what further research they did would have been taken to confirm Aristotle, at least broadly.
They did indeed think that the process of generation was gradual and epigenetic in some sense (when they talk about conception, they don't mean a specific point but an extended process in which materials are cooked together), but this is sometimes potentially misleading, since they held that there were definite stages. Talking about the whole process of generating a human being as hominization is not very accurate for Aquinas, for instance, because the earliest stages are not really hominization at all, but preparation for it, and the actual "coming into being of the human" itself happens suddenly when all the materials are prepared for it. Aquinas, as far as I know, doesn't commit to when this happens. He does mention that Aristotle held that male embryos are fully articulated at forty days and female embryos are fully articulated at ninety days, and that Augustine thinks this level of articulation happens at about forty-six days, but Aquinas himself doesn't seem to suggest any timeline at all for any of it.
Quickening plays almost no role in either Aristotelian or Catholic discussions of the matter. It arises in legal contexts, and it was not intended to convey metaphysical or biological baggage but simply to provide an easily identifiable legal cut-off (namely, when the baby recognizably moves on its own). Despite the name, it should not be confused with animation or ensoulment, which is a different concept; this is an extremely common confusion, but I have no idea why people keep conflating them, unless they just are confused by the name. In Aristotelian biology, animals have souls (that's what 'animal' means, in fact: soul-having), so on something like Aquinas's view, the preparatory stages would already have souls, just not a specifically human soul yet.
Everyone has of course always held in some sense that life begins at conception; that's what 'conception' meant in this context, the beginning of life, although, again, before post-medieval discoveries about egg fertilization, 'conception' always meant a process over time. Pius IX did not decree that life began at conception, because that was the universal view. Abortion had always been forbidden under canon law, but canon lawyers had always recognized that there were lots of uncertainties in the generation-process, especially early on, that were not easily accommodated by law, and also that in some accounts of it, like Aquinas's, the first stages of the embryo weren't human but merely preparatory to a human being. Because of this, at some point a cut-off was established: aborting after around seventeen weeks (if I recall correctly) was subject to automatic excommunication, whereas aborting prior to then was a less serious offense. Pius IX just made abortion at any stage of pregnancy subject to automatic excommunication; again, this was purely a legal matter. There was no decree of any kind about the nature of conception or the beginning of life. He was actually reinstating a previous decree by Sixtus V from 1588 that had not lasted very long for practical reasons. We don't know for sure the reasons why Pius IX did this, although it's a common view that he thought improvements in biological science made it more obvious that the cut-off was arbitrary.
