Why do we calculate the length of a pregnancy from the first day of a women’s last period?
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Because bleeding out of your genitals is a date that people remember and can easily track.
And in the years prior to ultrasound and blood tests it was the only method we had to date a pregnancy so it is the standard around which medical literature coalesced.
To be clear, the period date is not perfectly temporally offset from viable conception (ie. Women have different length cycles so the blastocyst can implant in the uterine wall a varying number of days after the period) but **it is accurate to within a few days in most cases, which is sufficient to provide good prenatal care.**
That said, in advanced healthcare systems you will get early ultrasounds that measure the foetus and from that we backdate a conception date and equivalent “day of last period”. And these are the dates that will go in the medical record.
In short, **first day of last period is a historical medical artefact that continues through cultural inertia and nowadays we use tests and imaging to backcalculate a “day of last period” that matches your conception day (for a normal cycle).**
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