You give the date your last period started, and a minute later someone turns it into a due date about nine months away. If it was counted from your period, it came from Naegele's rule, which one NHS trust says has been the basis of most due-date calculations for 200 years.
Below you'll find the Naegele's rule formula, worked examples, its assumptions, and how the American College of Obstetricians and Gynecologists (ACOG) sets due dates today.
Naegele's rule estimates a due date by adding 280 days (40 weeks) to the first day of your last period; the calendar shortcut of adding one year, subtracting three months and adding seven days lands on the same date or up to three days later. It is an estimate, and ACOG names a first-trimester ultrasound as the most accurate way to confirm or set it.
Key Takeaways
- Naegele's rule counts 280 days from the first day of your last period.
- It assumes a 28-day cycle with ovulation on day 14.
- A scan by 13 weeks and 6 days is the most accurate way to confirm or reset the date (ACOG).
- Few babies arrive on the due date: 4.4% at one NHS trust, 5% at another.
What Is Naegele's Rule?
Naegele's rule (also spelled Nägele's rule) turns the first day of your last menstrual period (LMP) into an estimated due date. ACOG's Committee Opinion 700, reaffirmed in 2025, describes the convention: the due date is 280 days after the first day of the LMP.
The count starts before conception: MedlinePlus, from the National Library of Medicine, measures gestational age from the first day of the last menstrual cycle. With ovulation on day 14, you're counted as two weeks pregnant around conception.
The rule is named for German obstetrician Franz Carl Naegele (1778 to 1851). A 2000 BJOG paper suggested it may have been misread, moving due dates earlier; a 2016 re-reading of his textbooks found he advised counting from the period's first day.
The assumptions behind it
ACOG notes that the count assumes a regular 28-day cycle with ovulation on day 14. It can't account for a misremembered period date, irregular cycle lengths or variable ovulation, and about half of women recall their LMP correctly. The rule fits less well if:
- Your cycles are irregular or differ from 28 days.
- You're unsure when your last period started. The NHS suggests asking a midwife or GP (family doctor), and ACOG says to date the pregnancy by ultrasound.
- You conceived through IVF, which ACOG dates from the embryo transfer.
The Naegele's Rule Formula
- 280-day count: first day of last period + 280 days.
- Calendar shortcut: first day of last period + 1 year − 3 months + 7 days.
A 2021 review on PubMed describes the rule as nine months and seven days added to the LMP. Months differ in length, so the versions can disagree: across every start date from 2020 through 2031, our script found the shortcut lands on the 280-day date or up to three days later. Counting days also avoids results like "February 30" from a May 30 start.
From an ovulation date, the same arithmetic gives 266 days (280 minus 14).
Our due date calculator applies Naegele's rule and adjusts it for your cycle length (21 to 35 days). It counts days (280 for a 28-day cycle), and its Ovulation option adds 266 days to an ovulation or conception date.
Naegele's Rule Examples
Made-up dates, counted in calendar days by script.
| First day of last period | Calendar shortcut | 280-day count |
|---|---|---|
| June 15, 2026 | March 22, 2027 | March 22, 2027 |
| March 28, 2026 | January 4, 2027 | January 2, 2027 |
| February 10, 2024 (leap year) | November 17, 2024 | November 16, 2024 |
Mid-year (June 15): add a year, subtract three months, add seven days, and you reach March 22, 2027, the same as the 280-day count.
Into the new year (March 28): subtracting three months from March 28, 2027 drops back to December 28, 2026, and seven days carry it to January 4, 2027, a 282-day span.
Leap-year February (February 10, 2024): February 29 falls inside the pregnancy, so the shortcut runs 281 days. From February 10, 2026, both methods give November 17, 2026.
Adjusting for cycle length: some calculators add a step Naegele's rule doesn't have. Ours adds 280 + (cycle length − 28) days, and the NHS due date calculator also asks you to adjust for a shorter or longer cycle.
| Usual cycle | Days added | Due date for a June 15, 2026 period |
|---|---|---|
| 24 days | 276 | March 18, 2027 |
| 28 days | 280 | March 22, 2027 |
| 32 days | 284 | March 26, 2027 |
| 35 days | 287 | March 29, 2027 |
ACOG's Committee Opinion 700 doesn't describe this adjustment; for unreliable period dates, it uses ultrasound.
How Are Due Dates Calculated Today?
Committee Opinion 700 keeps the LMP as a starting point and adds an early ultrasound check and an IVF method. ACOG says your provider should set the date once they have your LMP, a first reliable ultrasound or both, and discuss it with you.
First-trimester ultrasound
ACOG calls a first-trimester ultrasound, up to and including 13 weeks and 6 days, the most accurate method to establish or confirm how far along you are. It measures crown-rump length (CRL), head to bottom, to within plus or minus 5 to 7 days. See the dating scan for the appointment itself.
ACOG's Table 1, "Guidelines for Redating Based on Ultrasonography":
| Gestational Age Range* | Method of Measurement | Discrepancy Between Ultrasound Dating and LMP Dating That Supports Redating |
|---|---|---|
| ≤13 6/7 wk | CRL | |
| • ≤ 8 6/7 wk | More than 5 d | |
| • 9 0/7 wk to 13 6/7 wk | More than 7 d | |
| 14 0/7 wk to 15 6/7 wk | BPD, HC, AC, FL | More than 7 d |
| 16 0/7 wk to 21 6/7 wk | BPD, HC, AC, FL | More than 10 d |
| 22 0/7 wk to 27 6/7 wk | BPD, HC, AC, FL | More than 14 d |
| 28 0/7 wk and beyond† | BPD, HC, AC, FL | More than 21 d |
AC, abdominal circumference; BPD, biparietal diameter; CRL, crown-rump length; FL, femur length; HC, head circumference; LMP, last menstrual period. *Based on LMP. †ACOG says redating could mislabel a small baby who may be growth restricted, so decisions based on a third-trimester scan alone are especially problematic and need the whole clinical picture.
For example, a scan at 8 weeks that differs from your period date by more than 5 days supports moving the date. If you're unsure of your period date, ACOG treats the earliest CRL scan as the most reliable.
IVF due dates
For IVF, ACOG uses the embryo's age and the transfer date: 261 days after a day-5 transfer, 263 after a day-3 transfer (266 minus embryo age). A July 1, 2026 transfer gives March 19 or March 21, 2027. Royal Berkshire NHS Foundation Trust keeps the IVF clinic's date and doesn't change it after scans.
The calculator's IVF option counts from a day-3 or day-5 transfer with the same arithmetic.
Why your due date shouldn't keep changing
ACOG reserves later changes for rare circumstances, and a second-trimester scan should rarely override a first-trimester one. Third-trimester scans are the least reliable (plus or minus 21 to 30 days), and a pregnancy with no scan confirming or revising the date before 22 weeks is "suboptimally dated."
Two questions for your OB or midwife if a later scan suggests a new date:
- Is my official due date from my period or from a scan?
- Does this scan change my due date, or does it describe the baby's size?
How Accurate Are Due Dates?
Few babies arrive on the due date. Royal Berkshire NHS Foundation Trust's May 2026 leaflet puts it at 4.4%, fewer than 9 in 200, and University Hospitals Sussex NHS Foundation Trust at 5%; neither names its data.
The NHS says pregnancy normally lasts 37 to 42 weeks from the first day of your last period; MedlinePlus says 38 to 42. Royal Berkshire adds that most babies are born by 41 weeks.
ACOG's Committee Opinion 579 sets the labels: 37 and 38 weeks count as early term; full term starts at 39 and runs through 40 weeks 6 days, late term is week 41, and postterm starts at 42. For what happens next, see past your due date.
In one study ACOG cites, 40% of women given a first-trimester scan had their due date moved for a gap of more than 5 days, against 10% scanned in the second trimester.
Working Backward: From Due Date to Conception
Subtract 266 days from a due date and you reach the day the rule assumes you ovulated, 14 days after your period started. Working back from a March 22, 2027 due date gives June 29, 2026. Ovulation timing varies (ACOG), so treat that day as an estimate. Our guide to the question when did I conceive explains why the answer is a window of days.
Why 40 Weeks Is "9 Months" (Sort Of)
Divide 280 days by the average calendar month (365.25 ÷ 12, about 30.4 days) and you get about 9.2 months, with the first two weeks falling before conception. Our pregnancy weeks to months chart converts any week.
Frequently Asked Questions
What is Naegele's rule?
A way to estimate a due date: count 280 days (40 weeks) from the first day of your last period. ACOG names a first-trimester ultrasound as the most accurate way to confirm it.
What is the formula for Naegele's rule?
First day of your last period, plus one year, minus three months, plus seven days. It matches the 280-day count or lands up to three days later, because months differ in length.
Is Naegele's rule accurate?
It gives an estimate. It assumes a 28-day cycle with ovulation on day 14 (ACOG). Two NHS trusts say 4.4% and 5% of babies arrive on the due date itself.
Does Naegele's rule work with irregular periods?
Less well, since it assumes a regular 28-day cycle. If you're unsure of your period date, ACOG says to date the pregnancy by ultrasound, ideally by 13 weeks and 6 days.
How is the due date calculated for IVF?
From the embryo's age and the transfer date, says ACOG: 261 days after a day-5 embryo transfer, or 263 days after a day-3 transfer.
Can my due date change after an ultrasound?
Yes, usually early on. Before 14 weeks, ACOG's table supports redating if the scan differs from your period date by more than 5 to 7 days. Once your provider has set the date, ACOG reserves later changes for rare circumstances.
This article is for information only and does not replace medical advice from your OB or midwife. Your due date is an estimate, so ask your care team how yours was set.
How This Guide Was Made
The Babysential Team wrote this from the sources below, checked October 11, 2026, with AI-assisted drafting. The journal records support the history note only.
Sources
- Methods for Estimating the Due Date, Committee Opinion No. 700. ACOG, 2017, reaffirmed 2025
- Definition of Term Pregnancy, Committee Opinion No. 579. ACOG, 2013, reaffirmed 2025
- Due date calculator. NHS, reviewed 2024
- Baby due dates and how it affects my care. Royal Berkshire NHS Foundation Trust, reviewed 2026
- Preparing for the birth. University Hospitals Sussex NHS Foundation Trust, reviewed 2024
- Gestational age. MedlinePlus, U.S. National Library of Medicine, reviewed 2025
- Naegele's rule: a reappraisal. Baskett and Nagele, BJOG, 2000 (PubMed record)
- Naegele's rule revisited. Loytved and Fleming, Sexual & Reproductive Healthcare, 2016 (PubMed record)
- Naegele's rule and the length of pregnancy: a review. Lawson, Australian and New Zealand Journal of Obstetrics and Gynaecology, 2021 (PubMed record)
