This article is for educational purposes only and is not a substitute for personalised prenatal care. Always consult your obstetrician or midwife for your own due date and pregnancy timeline.
What is Naegele’s Rule?
Naegele’s Rule is the standard formula for calculating a pregnancy’s estimated due date (EDD): take the first day of the last menstrual period (LMP), add 9 calendar months and 7 days (equivalently: add 1 year, subtract 3 months, add 7 days). The result is the EDD — approximately 280 days, or 40 weeks, from the LMP.
Formally:
EDD = LMP + 280 days
or equivalently:
EDD = (LMP month − 3, LMP day + 7, LMP year + 1)
For example: if the LMP started on 1 April 2025:
- Subtract 3 months → 1 January
- Add 7 days → 8 January
- Add 1 year → 8 January 2026
The Pregnancy Due Date Calculator computes this for you instantly — input your LMP date and cycle length, and it returns your EDD, the likely fertilisation date, and the end of each trimester.
Where the 280 days comes from
The 280-day figure is not arbitrary. It corresponds to:
- 40 weeks of pregnancy
- 10 lunar months (10 × 28 days)
- 9 calendar months and 7 days
The rule was first published by the German obstetrician Franz Karl Naegele in 1812, based on clinical observation (and possibly folklore — the 10-lunar-month figure appears in many pre-modern traditions). It was adopted widely because it gave reliable results in most cases.
The ovulation assumption
Naegele’s Rule implicitly assumes that:
- Your menstrual cycle is 28 days long.
- Ovulation occurs on day 14 of that cycle.
- Fertilisation occurs on the same day as ovulation.
This means the first 14 days (from LMP to ovulation) are included in the 280-day count, even though no conception has occurred during that time. The actual conception-to-birth duration is closer to 266 days (38 weeks), not 280.
For women with cycles that differ from 28 days, the rule overestimates or underestimates the due date:
| Cycle length | Ovulation day | Adjustment |
|---|---|---|
| 21 days | Day 7 | EDD is 7 days earlier than Naegele |
| 28 days | Day 14 | No adjustment (standard) |
| 35 days | Day 21 | EDD is 7 days later than Naegele |
The Pregnancy Due Date Calculator applies this correction automatically — just enter your typical cycle length and it shifts the EDD accordingly.
Naegele vs. ultrasound dating
Modern obstetrics increasingly relies on first-trimester ultrasound dating rather than LMP for the official EDD. Here’s why:
LMP dating limitations
- Women may not remember their LMP exactly, especially with an irregular cycle.
- Cycle length varies from cycle to cycle, so the “28-day assumption” may not match any particular cycle.
- Implantation bleeding can be mistaken for a period, throwing off the LMP date.
Ultrasound dating
A crown-rump length (CRL) measurement at 7–13 weeks correlates tightly with gestational age (within ±5–7 days). By comparing the measured size to the expected size for a given gestational age, the sonographer can establish a more accurate EDD.
ACOG guidance: If the difference between the LMP date and the ultrasound date is:
- ≤7 days (first trimester): keep the LMP date
- 8–13 days (first trimester): revise to ultrasound date
- 14 days or more: always revise to ultrasound date
After 20 weeks, ultrasound biometry is less accurate for dating (± 3–4 weeks), so it is not used to revise an already-established EDD.
Why only 5% of babies arrive on their due date
The EDD is a median estimate — it sits in the middle of the expected range, not at the end. Labour onset is driven by a complex interaction of fetal readiness signals, maternal hormones, and prostaglandins. A normal pregnancy is considered to end anywhere between 37 weeks 0 days and 42 weeks 0 days:
| Gestational age | Classification |
|---|---|
| < 28 weeks | Extremely preterm |
| 28–33 weeks | Very preterm |
| 34–36 weeks | Late preterm |
| 37–38 weeks | Early term |
| 39–40 weeks | Full term |
| 41 weeks | Late term |
| ≥ 42 weeks | Post-term |
Studies of spontaneous labours (without induction) show:
- About 50% of first-time mothers deliver between 39 weeks 3 days and 41 weeks
- Only around 5% deliver on the exact EDD
- About 80% deliver within 2 weeks of the EDD in either direction
The role of the EDD in prenatal care
Even though the EDD is an approximation, it is indispensable for:
- Trimester dating: Providers use the EDD to schedule first-trimester screening (10–13 weeks), the 20-week anatomy scan, the gestational diabetes screen (24–28 weeks), and Group B strep testing (35–37 weeks).
- Growth monitoring: Fetal weight estimates and growth percentiles are calculated against the EDD.
- Induction decisions: Most providers offer induction between 41 and 42 weeks if labour has not started spontaneously.
- Legal and insurance records: The EDD is recorded at the first prenatal visit and used in all subsequent documentation.
Computing your own EDD
Use the Pregnancy Due Date Calculator to compute:
- Estimated due date (Naegele’s Rule, adjusted for cycle length)
- Estimated fertilisation date (LMP + cycle length − 14 days)
- End of each trimester (end of T1: week 13; end of T2: week 27; birth: week 40)
- Three upcoming predicted periods if pregnancy has not been confirmed
Input your LMP date and your typical cycle length. The calculator adjusts the EDD when your cycle differs from 28 days — giving you a more personalised estimate than the standard rule alone.
Summary
Naegele’s Rule — LMP plus 280 days — is a 200-year-old formula that remains the starting point for pregnancy dating. Its main limitation is the assumption of a 28-day cycle with ovulation on day 14. Tools that adjust for cycle length (like our Pregnancy Due Date Calculator) improve accuracy for women with irregular or non-standard cycles. First-trimester ultrasound remains the most accurate dating method when it is available.