Start at Day One?
Biologically, nobody knows they’re pregnant on Day 1 — it’s literally impossible to know until about 1-2 weeks at the earliest, if you are peeing on a stick literally every day.
Even then, it’s recommended for women to wait until a week or two after their first missed period for the most accurate results.
That can get into 6-week territory.
Then of course some women have irregular periods and miss them for different reasons and they may not even be able to use *that* as a reliable gauge.
So even under the “life begins at conception” theory, it’s only possible to determine that in retrospect, sometimes many weeks out.