LMP, due dates, etc.

Lacey • Proud C-section Momma of a bottle baby🍼 Working towards optimal mental and physical health.

ok so just wondering how the whole due dates based of LMP work. do you count from first day or the last day? or since I tracked my ovulation and had obvious IB would I count from those days? if I go with my first day of LMP I'm 7w4d. if I go from last day of LMP I'm 6w4d. if I do when I ovulated I'm 5w exactly. and if I do when I had IB I'm 3w5d. my period would have been Nov 18.

I've had bloodwork and maternity physical. doctors confirmed pregnant.

first ultrasound is Dec 20.