Why do we calculate the length of a pregnancy from the first day of a women’s last period?

Question

Why do we calculate the length of a pregnancy from the first day of a women’s last period?

In: 3216

Answer 1

because that’s the only date of the cycle known for sure. neither the date of ovulation, nor date of fertilization are known for sure, unless it is a rare single-time intercourse like in case of rape, but the observations and conclusions were made primarily on average women who have survived the first delivery already