Actually, it is possibly an important question as to which was the change in an aeon-long definition.
During the patrisic era, the understanding of 'conception' was analogized to a seed being planted in the fertile soil of the womb. One speaks traditionally of a woman conceiving, and applies the usage from the moment of implantion--a sign of which is the cessation of menses. One does not speak of the woman conceiving if egg and sperm join, but the embryo fails to implant and leaves her body, because the (very common) event is unobserved.
While I think the Latin church's conclusions on the matter are generally sound, I think they have been arrived at in too facile a manner, by accepting a medical redefinition of conception as fertilization--either conception = fertilization, or conception = implantation, was really a redefinition, since no distinction between the two was known until the application of microsopy to reproduction--without fully considering the patristic and Scriptural testimonies as the Fathers, evangelists, apostles and prophets understood them.
Under the new circumstances, may it be that a distinction like the formed/unformed (rightly rejected by St. Basil the Great as applied to children gestating in the womb) might rightly be applied to pre-implantation/implanted? That abortion (meaning of an implanted embryo or fetus) is murder, but that the destruction of an un-implanted embryo is a lesser sin? (It is the case that most embryos don't implant (!), and while it may be God's will to create many, many human beings who never have the chance of finding grace in Christ because they died as little spheres of 8 or 16 genetically distinctive human cells, more of them indeed than human beings who have walked the earth, it seems odd indeed to my still-passionate sinful human mind. So odd, indeed--as the Psalmist tells us, He does not wish to destroy the works of His hand--that the fact may matter to our moral understanding of the matter.)
(I'm Christian, but not Catholic.I'll leave ensoulment to G_d. Thank Him, He's got more wisdom than I. But - ?in that wisdom? - He has given me the chance to study embryology and to use the knowledge from gained from others. And He gave me the belief that I can't discriminate among the His images.)
Yes, neither "fertilization" nor "conception" were visualized until recently. In fact, I've always used them interchangeably as synonyms, and that's the way they were used in the pregnancy books I read in the early '70's.
Now, partly because of distructive research on extracorporeal embryos, we understand that there is a gap in time in vivo - and a possible greater disconnect in the lab - between fertilization/conception and implantation.
But, the embryo is initiated at the joining of the sperm and the oocyte. From penetration of the zona pellucida, in fact, there is polarization that seems to persist in the normal course of events unless the cells of the embryo are separated from one another.
I think it was Martin Gardner wrote about an analogy that I like: that science could be compared to the application of better and better cameras. Just because I can get a sharper image or take a video of the event (or even interfere with the help of cameras and other technology) doesn't change the nature of the event itself.