38 wks conceprion? 49 weeks lmp

[Unknowingly1]

To find out approx conception date, do you count back 38 weeks from due date?

Also, if doctors have given your due date via ultrasound and you count back 40 weeks is this roughly the date you had your period, guaranteed? (Cant remember my LMP)

Yes, if you've been given a due date from your scan, count back 38 weeks from that to find out your approx conception date.

Then add another 2 weeks on for the estimated start date of that cycle (LMP) although that won't be very accurate if your cycles are longer or shorter than average. The LMP date is based on a very average 28 day cycle with ovulation bang in the middle so no, not guaranteed. Mine is about 10 days out.