How Is a Due Date Calculated? Naegele’s Rule Explained

Naegele’s Rule

Named after Franz Karl Naegele, a German obstetrician who popularised it in the early 1800s, the rule survives because it can be done in your head.

The starting point is the first day of your last menstrual period, abbreviated LMP, meaning the first day of proper bleeding rather than spotting. Not the day it ended, and not the day you think you conceived.

One small wrinkle worth knowing: because calendar months vary in length, the shortcut and the 280-day count can differ by a day or two depending on which months your pregnancy spans. Clinicians and calculators use the 280-day count, which is the more precise of the two.

Worked Example

Last menstrual period began 10 January 2026.

Using the shortcut

• 10 January + 7 days = 17 January 2026
• 17 January − 3 months = 17 October 2025
• 17 October + 1 year = 17 October 2026

Using 280 days

Counting 280 days forward from 10 January 2026 also lands on 17 October 2026, which is a useful check that the shortcut behaved.

That date represents 40 weeks of gestation. Week 37 begins on 19 September 2026, which is the point at which a birth stops being considered premature, and 42 weeks falls on 31 October 2026.

The Two Assumptions Behind It

Naegele’s rule quietly assumes two things about your body, and they are worth examining because a great many people do not match them.

The first is that your cycle is 28 days long. The second is that you ovulated on day 14. Normal cycles run anywhere from 21 to 35 days, and ovulation timing shifts accordingly, so on a 35-day cycle you likely ovulated closer to day 21 and the pregnancy is a week younger than the calculation assumes.

There is also a third quiet assumption hiding in the method: that you know your LMP date accurately. Irregular bleeding, spotting mistaken for a period, recently stopping hormonal contraception, or simply not having noted it all undermine the starting point. The formula is only as good as the date you feed it.

Adjusting for Your Cycle Length

The correction is straightforward. Add the difference between your cycle length and 28 days.

Applied to the example above, someone with a 32-day cycle and an LMP of 10 January 2026 would have an adjusted due date of 21 October 2026 rather than 17 October.

Four days sounds trivial until it sits near the boundary of a decision about induction, which is one reason accurate dating matters clinically rather than just satisfying curiosity.

Why Ultrasound Usually Wins

If your dating scan disagrees with your calculated date, the scan is generally the one to trust.

An ultrasound in the first trimester, up to and including 13 weeks 6 days, is the most accurate method available for establishing gestational age, with an accuracy of roughly plus or minus 5 to 7 days. Measuring crown-rump length in that window narrows it further, to about plus or minus 5 days. Early embryos grow at a remarkably consistent rate, which is what makes the measurement so reliable.

Accuracy degrades as pregnancy progresses, because growth rates begin to diverge between individual babies. A scan between 14 and 20 weeks carries a margin closer to plus or minus 8 days, and third-trimester scans are poor for dating, which is why an early scan is worth attending even when you are confident about your dates.

Professional guidance is that a well-performed early ultrasound is more accurate than even a certain menstrual date. Providers typically revise the due date if the scan differs from the LMP calculation by more than about 5 to 7 days in the first trimester.

What the Date Actually Predicts

It is better understood as the centre of a range than as an appointment.

Only about 4% of babies arrive on their estimated due date. Around 80% arrive within the two weeks either side of it, which is a four-week spread. First pregnancies tend to run slightly longer than subsequent ones.

The categories obstetric guidance uses reflect this spread rather than treating 40 weeks as a target. Birth from 37 weeks is considered early term, 39 to 40 weeks full term, 41 weeks late term, and beyond 42 weeks post-term. Most of that range is entirely normal.

Which makes the phrase “estimated due date” worth taking literally. Expect a window of a few weeks, and treat the specific date as a scheduling convenience rather than a prediction.

Gestational Age vs Fetal Age

A quirk of the system confuses almost everyone at some point: pregnancy is counted from your last period, which is roughly two weeks before you conceived.

So at the moment of conception you are already considered two weeks pregnant, and by the time a missed period prompts a test you are typically around four weeks. Gestational age, the number used in all clinical care and in every due date calculation, runs about two weeks ahead of the actual age of the embryo.

Fetal age, sometimes called conceptional age, is gestational age minus two weeks. It is occasionally quoted in embryology contexts and almost never in clinical practice. If two sources give you ages a fortnight apart, this is usually why.

Frequently Asked Questions

How accurate is a due date?

As a single date, not very: roughly 4% of babies are born on it. As the centre of a range it works well, with about 80% of births falling within two weeks either side. Dating established by a first-trimester ultrasound is accurate to around 5 to 7 days, better than any calendar calculation.

What if I do not know my last period date?

An early ultrasound will date the pregnancy for you, and in the first trimester it does so more accurately than the LMP method anyway. This is common with irregular cycles or after stopping hormonal contraception, and it is not a problem for your care.

Does a long cycle change my due date?

Yes. Add the difference between your cycle length and 28 days. A 35-day cycle pushes the due date a week later, because ovulation happened around day 21 rather than day 14. Tell your provider your usual cycle length so they can factor it in.

Why is pregnancy dated from before conception?

Because the first day of a period is a date most people can identify, while the day of conception usually is not. The convention makes gestational age about two weeks longer than the embryo’s actual age, which is why you are considered four weeks pregnant around the time of a missed period.

Will my due date change after a scan?

It may. Providers generally revise the date if an early scan differs from the LMP calculation by more than roughly 5 to 7 days, since first-trimester measurements are the more reliable of the two. A revised date is a normal part of care rather than a sign that anything is wrong.

How is the due date worked out for IVF?

Far more precisely, because the date of egg retrieval or embryo transfer is known exactly. The calculation runs from that date, adjusted for the age of the embryo at transfer, which removes the guesswork about ovulation entirely. Your clinic will provide the date.

What happens if I go past my due date?

Going a little past 40 weeks is common and usually uneventful. Providers typically increase monitoring after 41 weeks and discuss induction somewhere between 41 and 42 weeks, since risks rise beyond that point. The specifics vary by country and by your individual circumstances, so follow your own provider’s guidance.

The Bottom Line

Add 280 days to the first day of your last period, or use Naegele’s shortcut of plus seven days, minus three months, plus a year. Adjust for a cycle longer or shorter than 28 days, and expect an early ultrasound to refine or overrule the result, because a first-trimester scan is accurate to within about five days. Then hold the date loosely. Roughly one baby in twenty-five arrives on it, and a four-week window around it is the honest expectation. Work out your own estimate with the due date calculator, and let your midwife or doctor confirm it at your dating scan.

Sources and Further Reading

• American College of Obstetricians and Gynecologists. “Methods for Estimating the Due Date.” Committee Opinion, 2017.
• Society of Obstetricians and Gynaecologists of Canada. “Guideline No. 388: Determination of Gestational Age by Ultrasound.”
• UK NHS – guidance on dating scans and pregnancy length.