How Pregnancy Due Dates Are Calculated (Naegele's Rule)
Due dates are estimated from your last period using Naegele's rule (LMP + 280 days). Learn how it works, cycle-length adjustments, and ultrasound accuracy.
Your due date is an educated guess about when a baby who hasn't been conceived yet will arrive, calculated from a period you may barely remember. It's surprisingly reliable — but the arithmetic and the vocabulary trip most people up. Here's how the standard estimate works, why "you're four weeks pregnant" often means "conception happened two weeks ago," and when ultrasound overrules the calendar.
Naegele's rule
The classic estimate, credited to 19th-century German obstetrician Franz Naegele:
Due date = first day of last menstrual period (LMP) + 280 days
Or equivalently: LMP + 9 months + 7 days. That's a 40-week estimated pregnancy — even though a baby actually spends only about 38 weeks developing.
LMP vs conception: the 2-week offset
This is the piece that confuses everyone. In a textbook 28-day cycle:
- Day 1 — first day of the last menstrual period (this is week 0 of "pregnancy")
- ~Day 14 — ovulation and conception
- ~Day 21 — implantation
- Day 28 — missed period; you're already "4 weeks pregnant"
Gestational age is counted from LMP, not from conception. So "8 weeks pregnant" means roughly 6 weeks since conception. The Conception Calculator reverses the arithmetic to find a likely conception date, and the Ovulation Calculator estimates your fertile window for future cycles.
A worked example
LMP: January 3, 2026
- Add 280 days → October 10, 2026 (Naegele estimate)
- Add 9 months + 7 days → October 10, 2026 ✓
That's the estimated due date (EDD). Full-term is anywhere from 37 to 42 weeks; only about 4% of babies arrive on the exact due date, and roughly 80% arrive within the two weeks around it.
Cycle-length adjustments
Naegele's rule assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the timeline:
| Your cycle length | Adjustment to EDD |
|---|---|
| 21 days | Subtract 7 days |
| 24 days | Subtract 4 days |
| 28 days | No adjustment |
| 32 days | Add 4 days |
| 35 days | Add 7 days |
| 40 days | Add 12 days |
The Pregnancy Calculator applies this correction automatically when you enter a non-standard cycle length.
Ultrasound dating: when it wins
Early ultrasound — especially between 8 and 13 weeks — measures the embryo's crown-rump length and estimates gestational age with high accuracy:
| Ultrasound timing | Typical accuracy of dating |
|---|---|
| 6–8 weeks | ±3 days |
| 8–13 weeks | ±5–7 days |
| 14–20 weeks | ±7–10 days |
| 20–30 weeks | ±14 days |
| 30+ weeks | ±21+ days |
If the ultrasound EDD differs from the LMP EDD by more than the accuracy window (usually 5–7 days in the first trimester), most guidelines re-date the pregnancy to the ultrasound. After 22 weeks, LMP-based dating is usually kept even if ultrasound disagrees — the ultrasound window is too wide to overrule a known LMP.
Why 280 days?
Empirically, term singleton pregnancies from a known LMP average about 280 days, with a standard deviation of ~10 days. Newer research suggests the true average may be slightly longer — closer to 283–288 days for first pregnancies — but 280 remains the clinical standard because it's simple, historical, and close enough for planning.
FAQ
How accurate is Naegele's rule for calculating a due date? It puts most pregnancies within ±10 days of actual delivery. Only about 4% deliver on the exact EDD; 80% deliver within two weeks either side.
Why does the doctor say I'm 6 weeks pregnant when I conceived 4 weeks ago? Gestational age is counted from the first day of your last period, not from conception — a two-week head start baked into the vocabulary. It's the clinical standard everywhere.
Can an ultrasound change my due date? Yes, especially in the first trimester. If a first-trimester scan disagrees with your LMP-based due date by more than about a week, the ultrasound date typically becomes the official EDD.
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*This is general information, not medical advice — consult your healthcare provider for pregnancy care. Try the free Pregnancy and Conception calculators to see your timeline.*