How the due date calculator works
Naegele's rule — and its adjustments
The classic method: LMP + 280 days (40 weeks), assuming a 28-day cycle with ovulation on day 14. For other cycle lengths, the adjustment shifts the estimate by (cycle length − 28) days — a 35-day cycle adds ~7 days. Conception-date mode adds 266 days (38 weeks from conception); IVF mode uses the transfer date plus the embryo's age — the most precise of the three because the conception moment is known exactly.
What the date actually is
The due date is the center of a distribution, not an appointment: ~50% of births land within a week of it, ~80% within two weeks, and about 4% on the day itself. First babies average slightly later; the honest reading is "an estimated 40-week midpoint, ±2 weeks of normal."
Ultrasound dating overrides the math
A first-trimester ultrasound (before ~14 weeks) measures the embryo directly and is more accurate than any date-based method — differences beyond ~5–7 days trigger a formal date change on your chart. This is expected and routine, not a contradiction: the calculator gives you the calendar's best guess before the scan refines it.
Week counting, demystified
Obstetric weeks count from the LMP — so at conception you are already "2 weeks pregnant." Gestational age (from LMP) = fetal age + ~2 weeks. The calculator shows both the current week and days-remaining so the clinic's numbers always match yours.