That is, given our best medical and philosophical understandings of what makes something not just a living organism but a person, there is no way to establish at just what point during the gestation a fertilized ovum becomes a human being.
be better if rewritten as:
That is, given our best medical and philosophical understandings of what makes something not just a living organism but a person, there is no way to establish at just what point during the gestation a fertilized ovum becomes a person.
I like the second. However, do you wish to use the word gestation?
Although it does mean the time between conception and birth it is commonly synonymous with pregnancy.
As a pregnancy only begins after implantation of the embryo I think something like
That is, given our best medical and philosophical understandings of what makes something not just a living organism but a person, there is no way to establish at just what point between conception and birth a fertilized ovum becomes a person.
might be more specific as it allows for the “person from conception” viewpoint as well as the “person only after the head exits the birth canal” viewpoint.
The first version boils down to: “Given blah blah blah about a person, blah blah blah about a human being.”
The second version boils down to: “Given blah blah blah about a person, blah blah blah about a person.”
The second one has a very good logical structure, but the first one lacks this logical structure because you changed from talking about a “person” to talking about a “human being”. To fix this, you need to put “human being” in both parts of the first version.
Then, once you’ve done that, here’s what my answer to the OP would be: “Human being” is a species and is determined by genetics. That means at fertilization, or maybe even before fertilization. “Person” is what makes us more than organisms, and so I’d say go with the (repaired) first version.