Every pregnancy care plan starts with one question: when is the baby due? Doctors, midwives and nursing students answer it with Naegele’s rule, a quick bedside method that turns one date, the first day of the last menstrual period, into an expected date of delivery (EDD). In this guide you will learn the formula, see worked examples, and understand when the rule works and when it does not.
What Is Naegele’s Rule?
Naegele’s rule is a simple calculation that doctors, midwives and nursing students use to predict when a baby is likely to be born. All it needs is one date: the day the woman’s last period began, known as the LMP. Using that starting point, the rule gives an expected date of delivery (EDD). It is named after Franz Karl Naegele, the 19th-century German obstetrician who is credited with it.
The rule assumes:
A regular menstrual cycle of about 28 days
Ovulation around day 14 of the cycle
A total pregnancy length of about 280 days (40 weeks), counted from the LMP
Naegele’s Rule Formula
EDD = LMP + 9 months + 7 days
The steps:
Find the date on which your last period began. This is day one of your cycle.
Add 7 days to that date.
Subtract 3 months.
Add 1 year.
Shortcut by month:
LMP in April to December: add 7 days, subtract 3 months, keep the following year.
LMP in January to March: add 7 days, add 9 months, keep the same year.
Counting forward this way lands on almost the same date as counting 280 days (40 weeks) from the first day of the last period.
Worked Examples
Example 1: LMP on 10 January 2026
Add 7 days: 10 Jan becomes 17 Jan
Subtract 3 months: 17 Jan becomes 17 Oct (2025)
Add 1 year: 17 Oct 2026
EDD = 17 October 2026
Example 2: LMP on 25 October 2025 (date crosses into the next month)
Add 7 days: 25 + 7 = 32. October has 31 days, so this becomes 1 November
Subtract 3 months: 1 November becomes 1 August
Year: 2026
EDD = 1 August 2026
Tip: when adding 7 days pushes the date past the end of the month, use the actual number of days in the LMP month, then move to the next month.
Adjusting for Cycle Length
The standard rule assumes a 28-day cycle. If a woman’s cycles are regularly shorter or longer, ovulation happens earlier or later, and the EDD shifts.
Example: LMP is 10 January 2026 and the cycle is 35 days.
Standard EDD: 17 October 2026
Adjustment: 35 − 28 = 7 days
Adjusted EDD: 24 October 2026
How to Calculate Gestational Age from the LMP
Gestational age is the time passed since the first day of the LMP, expressed in weeks and days.
Example: LMP was 10 January 2026 and today is 14 March 2026.
Days between the two dates: 63
63 ÷ 7 = 9 weeks 0 days
Limitations of Naegele’s Rule
Naegele’s rule is quick, but it is only an estimate. It becomes less reliable when:
Cycles are irregular or the woman does not remember her LMP
Conception happened soon after stopping hormonal contraception or during breastfeeding, before regular periods returned
The pregnancy was conceived through IVF, because the embryo transfer date allows doctors to date the pregnancy far more precisely than the LMP.
Ultrasound findings disagree with the LMP date
In these cases, clinicians rely on an early ultrasound, and a first-trimester crown-rump length (CRL) measurement is generally considered the most accurate way to date a pregnancy. If the ultrasound date differs from the LMP date by more than the accepted margin for that gestational age, the EDD is usually revised.
The American College of Obstetricians and Gynecologists (ACOG) advises that a first-trimester ultrasound is the most accurate way to establish or confirm a due date, and it considers a pregnancy without a scan-confirmed EDD before 22 weeks to be suboptimally dated. You can read the full guidance in ACOG Committee Opinion 700: Methods for Estimating the Due Date.
EDD Is an Estimate, Not a Deadline
Only a small percentage of babies (roughly 4 to 5%) arrive exactly on the EDD. Most are born within a couple of weeks either side. Think of the EDD as the centre of a window, not a fixed appointment.
According to the NHS due date calculator, pregnancy normally lasts between 37 and 42 weeks from the first day of the last period, and a 12-week dating scan can estimate how far along you are more accurately.
Quick Way: Use an EDD Calculator
Doing the calculation by hand is important for exams, but for everyday use a calculator is faster and avoids month-end mistakes. Try our free EDD calculator to get your expected date of delivery in seconds.
Frequently Asked Questions
What is the formula for Naegele’s rule? Take the date your last period started, move it forward by 7 days, go back 3 months, and then move ahead by one year.
How many days does Naegele’s rule assume? It assumes a pregnancy of 280 days (40 weeks) from the LMP.
Is Naegele’s rule accurate? Yes, for the most part. It gives a dependable estimate when a woman has regular 28-day cycles and remembers her LMP clearly. If the cycles vary or the LMP date is uncertain, an ultrasound scan is the better way to date the pregnancy.
Does Naegele’s rule work for irregular periods? Not reliably. An early ultrasound is preferred in that case.
What is the difference between EDD and gestational age? EDD is a future calendar date (the expected day of birth). Gestational age is how many weeks and days have passed since the LMP.
Who created Naegele’s rule? Franz Karl Naegele, a German obstetrician, in the 19th century.
Naegele’s Rule & EDD Knowledge Quiz
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Conclusion
Naegele’s rule (LMP + 7 days − 3 months + 1 year) is one of the first formulas every nursing and midwifery student learns. It is simple and fast, but it works best with regular cycles and a known LMP. Always confirm dating with a healthcare provider and, when needed, an ultrasound.
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Disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice.
