Two dating systems, one winner
LMP-based dating (Naegele's rule) starts the clock from a date you remember imperfectly about an event that varies cycle to cycle. First-trimester ultrasound measures the embryo directly, accurate to ±5–7 days. When they disagree meaningfully, the ultrasound wins and the due date moves — not because the first estimate failed, but because better information arrived.
The re-date rules, briefly
Providers re-date when the scan (before 14 weeks) differs from LMP dating by more than ~7 days; after 14 weeks, the tolerance widens because growth variation grows. The FIRST ultrasound date sticks — later scans show growth, not gestational age, and re-dating from them introduces error rather than removing it.
Living with a two-week window
The date is the center of a distribution where 80% of births land within ±10 days. Nursery-ready by 37 weeks, work coverage through 42, family travel planned with flex — the window is the plan. Babies read the due date as a suggestion; the arithmetic agrees with them.