I searched for it online. Whether they know and I can’t find it or if they do not yet know, I don’t know, although I suspect the latter.
We know it happens. Why is the timing so important to you?
There are several articles in Wikipedia: “Human Fertilization,” “Zygote,” “Blastomere,” etc.
Before we even get to DNA, there are numerous steps in fertilization (see that article). Then the walls of the nuclei of the two cells have to dissolve, the DNA of each has to unravel, then be cut into pieces, and then re-assemble. None of this is instantaneous–it takes time.
Under the “Zygote” article: “After approximately 30 hours from the time of fertilization, fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres.”
[pronuclei come from the sperm and the egg, each with half the number of chromosomes necessary for new DNA to form.]
And of course this is biology, so timing depends on individuals. But 30 hours is probably a good estimate. Thus many Catholic hospitals have given “morning after” pills–although this has been very contentious.
So what? As you probably know many pro-life advocates use the argument that at conception a new, individual is formed with its own DNA. Well, yes, but that takes about 30 hours. And as you can see from the “human fertilization” article, there are a LOT of steps. At which step do you draw a line and say “OK, at this step we have conception.” And we haven’t even talked about the implantation of the new cells on the uterus wall, which takes a week or so, and the fact that up to 80% of the fertilized cells never implant and die. So that’s a huge number of deaths of persons–as they would be considered by anyone who believes human life begins at “conception” (whatever that means!). And we’re not talking miscarriages in the common use of the word here, which is what many pro-life advocates re-direct to. You most likely wouldn’t even be aware of the cells not implanting and dying.
All I’m trying to do here is simply point out that it’s complicated, with multiple steps over a fairly long period of time. None of which was known in 1869. Now of course you could avoid all the messiness and say “Conception begins at step #1, despite lack of individual DNA, etc. etc.” which is what the Church has done. And I’m NOT arguing that they’re wrong or can’t do that. But I would point out that if someone argues “Oh, it changed in 1869 because they followed the science…” then a logical deduction would be they should “follow the science” again and change again. But of course you don’t HAVE to make that argument–but many do.