Naegele's Rule
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Naegele's rule is the standard formula for calculating a pregnancy's estimated due date (EDD): add 280 days (40 weeks) to the first day of the last menstrual period (LMP). An equivalent formulation is to add one calendar year, subtract three months, and add seven days to the LMP date. The rule was published by German obstetrician Franz Karl Naegele in 1812 and remains the universal clinical standard for estimating due dates more than 200 years later.
The formula is based on the observation that most pregnancies last approximately 280 days from the LMP in a woman with a standard 28-day cycle. Because ovulation and conception typically occur around day 14 of the cycle, the actual fetal development period is about 266 days (38 weeks) - but gestational age, the universal clinical measure, starts from the LMP. For cycles longer or shorter than 28 days, the EDD is adjusted by the difference from 28 days.
Naegele's Rule explained to a beginner
Think of Naegele's rule as a pregnancy countdown timer with a built-in 2-week head start. You set the timer on the first day of your last period - before you were even pregnant. Then you count forward 280 days. The extra 2 weeks at the start account for the time between your period and when ovulation and conception actually happen. It's a clever workaround for the fact that nobody knows the exact moment they conceived, but almost everyone knows when their last period started.
When to use Naegele's Rule
Use Naegele's rule as the starting point for any pregnancy due date calculation. It applies when you know your LMP date and have a roughly regular cycle. If your cycle differs significantly from 28 days, adjust by the difference: a 35-day cycle means your EDD is 7 days later than the basic formula gives. The rule is superseded by first-trimester ultrasound if the two methods differ by more than 7 days - in that case, the ultrasound measurement takes precedence.
Worked examples for Naegele's Rule
This table quickly gives you the overview you need to understand Naegele's Rule and its most important comparisons.
| LMP date | Naegele formula (+280 days) | Estimated due date |
|---|---|---|
| January 1, 2026 | + 280 days | October 8, 2026 |
| March 15, 2026 | + 280 days | December 20, 2026 |
| June 1, 2026 | + 280 days | March 8, 2027 |
| September 10, 2026 | + 280 days (+ 2 days for 30-day cycle) | June 19, 2027 |
Common pitfalls
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Women with irregular cycles, PCOS, or cycles longer than 35 days will find LMP-based dating less reliable, and their due date should be confirmed or adjusted by first-trimester ultrasound. The rule also does not account for IVF pregnancies, where the exact transfer date is known and gives a more precise estimate.
Frequently asked questions about Naegele's Rule
How accurate is Naegele's rule?
Naegele's rule is accurate to within about 2 weeks for most women with regular cycles. Only about 5% of babies are born on the exact EDD. First-trimester ultrasound improves accuracy to within 5-7 days by measuring actual embryo size rather than relying on cycle assumptions. Most providers confirm or adjust the Naegele estimate with an early scan.
What if my cycle is not 28 days?
Adjust the due date by the difference between your cycle length and 28 days. A 30-day cycle means ovulation happens around day 16 instead of day 14, so your EDD shifts 2 days later. A 25-day cycle moves it 3 days earlier. The pregnancy calculator on this page applies this adjustment automatically when you enter your cycle length in the Last Period mode.
Is Naegele's rule still used today?
Yes - it's still the universal starting point for obstetric dating worldwide. ACOG, the WHO, and virtually every maternity care system begin with Naegele's rule and then adjust using ultrasound if the two methods differ by more than the allowed tolerance (7 days at under 14 weeks, 10 days at 14-28 weeks, 14 days after 28 weeks).
Quiz: how well do you know Naegele's rule?
1. According to Naegele's rule, how many days are added to the LMP to estimate the due date?
2. If a woman's LMP was January 1, what is her estimated due date using Naegele's rule?
3. A woman has a 35-day cycle. How should her due date be adjusted from the basic Naegele calculation?