Naegele's rule (1812, still surprisingly accurate) plus a real picture of where you are right now — week, trimester, and what's happening next.
Only about 4% of babies are actually born on their predicted due date. About 80% arrive within two weeks of it, which is what "due date" really means — the middle of a window, not a deadline.
We use Naegele's rule (last menstrual period + 280 days), adjusted for cycle length when you tell us yours. First-trimester ultrasound dating is more accurate than LMP, so if you have an ultrasound date, we'll use that instead.
What we don't do: pretend the date is medical advice, factor in twins or IVF without your input, or guess at cycle length. Source: ACOG Committee Opinion on Estimating Due Date.