Given a due date, estimate when conception (and your last period) likely occurred.
It subtracts 266 days from the due date to estimate the conception date, and 280 days to estimate the first day of your last menstrual period (LMP) — the reverse of the standard 40-week dating convention.
Conception is assumed to happen about 14 days after the LMP (around ovulation in a typical 28-day cycle), so 280 − 14 = 266 days separates conception from the due date.
The 2nd trimester is estimated to start 13 weeks after the estimated LMP, and the 3rd trimester 27 weeks after — both derived from the same backward-dated LMP, so they shift together if your due date changes.