The first thing to know about due dates is that the math is from 1812. A German obstetrician named Franz Naegele looked at a small Dutch sample and proposed: take the first day of the last menstrual period, add a year, subtract three months, add seven days. We still use it.
Naegele's rule assumes a 28-day cycle and a 14-day luteal phase. Both vary between people and between cycles. Real ovulation can shift the actual conception date by a week or more in either direction.
How accurate is "term"?
Modern obstetrics has split the old "term" into four buckets:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post term: 42 weeks 0 days and beyond
Most healthy first-time pregnancies deliver between 39 and 41 weeks. Only about 4% land exactly on the calculated due date.
Why first-trimester ultrasounds are more accurate
A crown-rump-length measurement at 8–13 weeks is the single most accurate dating tool we have, accurate to within 5–7 days. If your dating ultrasound shifts your due date by more than a week, doctors will use the ultrasound date going forward.