Conception date VS first day of your last period.

Sarah ā€¢ Married šŸ’ DOGMOM šŸ¾ Mommy šŸ’™šŸŽ€

Does anyone know why we don't calculate how far along we are with the date we conceived or around the date we conceived & why we do it with the first day of our last period? Cause technically weren't not pregnant from the first day of our last period... or at least it's unlikely. My husband was asking how far along I am & I said 7 weeks & he said but that doesn't make sense cause you said you ovulated December 24th... I didn't know how to explain it, I just said, the DR's have always told me to calculate it from the first day of your last period.