How Is Due Date Calculated? Naegele's Rule Explained
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: How is due date calculated? Most providers add 280 days (40 weeks) to the first day of your last period. This is Naegele's rule. If you know when you conceived, add 266 days instead. An early ultrasound then confirms the date or replaces it if the two differ by more than a set number of days.
This guide explains the formula, shows worked examples, covers why online calculators sometimes disagree, and explains when a scan changes your date. To skip the maths, use our free due date calculator.
The due date formula in one line
Due date = first day of your last menstrual period (LMP) + 280 days.
That's it. 280 days is 40 weeks. Doctors and midwives call this your estimated due date (EDD), and the NHS calculator uses the same starting point: the first day of your last period.
You can also do it by hand with Naegele's rule, named after the 19th-century German obstetrician Franz Naegele:
- Write down the first day of your last period.
- Add 7 days.
- Count back 3 months.
- Add 1 year.
Worked example: your last period started on 1 January 2026.
- 1 January + 7 days = 8 January
- Back 3 months = 8 October
- Due date: 8 October 2026
Adding 280 days to 1 January 2026 gives the same answer. Because months are different lengths, the two methods sometimes land a day apart. That doesn't matter, since both are estimates.
Other starting points
| What you know | Formula | Example |
|---|---|---|
| First day of last period (28-day cycle) | LMP + 280 days | 1 Jan 2026 → 8 Oct 2026 |
| First day of last period (longer or shorter cycle) | LMP + 280 days ± (cycle length − 28) | 35-day cycle: LMP + 287 days |
| Conception or ovulation date | Conception + 266 days | 15 Jan 2026 → 8 Oct 2026 |
| IVF, day-5 embryo transfer | Transfer + 261 days | See our IVF due date guide |
| IVF, day-3 embryo transfer | Transfer + 263 days | See our IVF due date guide |
| First-trimester scan | Based on the baby's crown–rump length | Set by your sonographer |
For the conception method in detail, see the due date calculator from conception.
Why doctors count from your last period
It can feel strange that pregnancy is dated from a period, roughly two weeks before you conceived. There are practical reasons:
- Almost everyone knows it. The first day of bleeding is easy to notice and remember. Ovulation and conception are not.
- It's a shared standard. Every midwife, doctor, scan report and app uses the same "gestational age", so a "20-week scan" means the same thing everywhere.
- It matches how the research was done. Growth charts, screening windows and term dates were all built on gestational age from the LMP.
The trade-off is that weeks 1 and 2 happen before you're pregnant. Our guide on why pregnancy calculators add 2 weeks explains this in more detail.
Why calculators give different dates
If you've tried three calculators and got three answers, it's usually one of these:
- Cycle length. Naegele's rule assumes a 28-day cycle with ovulation on day 14. Some calculators let you enter a longer or shorter cycle and shift the date. Others ignore it.
- Counting conventions. Some tools count the LMP as day 0, others as day 1, which moves the result by one day.
- Starting date. An LMP, a conception date and a transfer date can each give slightly different results, because ovulation timing varies.
- Rounding of months. Doing Naegele's rule by hand can land a day off from 280 days.
A difference of a day or two isn't worth worrying about. A difference of a week or more usually means the inputs were different, often the cycle length.
When an ultrasound changes your due date
Your LMP date is a starting point. The date that goes in your notes usually comes from a scan.
The American College of Obstetricians and Gynecologists (ACOG) says a first-trimester ultrasound, up to 13 weeks and 6 days, is the most accurate way to confirm or set gestational age. In England, the NHS offers a dating scan at around 10 to 14 weeks to see how far along you are.
ACOG's Committee Opinion 700 gives cut-offs for when the scan date should replace the LMP date:
| Gestational age at scan | Change the due date if the scan and LMP differ by more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks and later | 21 days |
The margins get wider because babies vary more in size as pregnancy goes on, so later scans are less precise for dating. ACOG also says a pregnancy without an ultrasound confirming the due date before 22 weeks should be considered "suboptimally dated". For IVF pregnancies, the date from the transfer is used instead of a scan.
Once your due date is set from an early scan, it usually stays fixed. Later scans check growth, not dates. Read more in can your due date change?
Your due date is a window, not a deadline
The NHS says only about 1 in 20 babies arrive on their due date. Most babies arrive between 37 and 42 weeks. ACOG's definitions are:
- 37 to 38 weeks + 6 days: early term
- 39 to 40 weeks + 6 days: full term
- 41 weeks: late term
- 42 weeks and beyond: post-term
That's why the NHS suggests having your hospital bag packed at least 3 weeks before your due date. If you go past 40 weeks, your midwife or doctor will talk to you about monitoring and induction.
How to work out your own due date
- Find the first day of your last period. Check your calendar or period app.
- Note your usual cycle length if it isn't close to 28 days.
- Enter both in the due date calculator, or use 280 days and adjust.
- Check your current week with How Many Weeks Pregnant Am I?.
- Book your first appointment. See the pregnancy appointment schedule for when your dating scan is likely.
- Update your date after the scan if your provider changes it.
If you don't know your LMP, had irregular cycles, or were recently using hormonal contraception or breastfeeding, the scan will be the main way to date your pregnancy. Our guide to how accurate pregnancy calculators are explains when to trust which date.
Follow your due date in Momi
Momi estimates your due date from the first day of your last period or a known conception date. From there, it follows your pregnancy week by week, with a progress ring showing how many weeks you have to go and your baby's size this week. You can log appointments, keep a diary and save your scan photos in the same place.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: due date calculator · IVF due date calculator · is pregnancy 9 or 10 months?
Sources
- ACOG, Committee Opinion 700: Methods for Estimating the Due Date (reaffirmed 2025)
- NHS, Pregnancy due date calculator
- NHS, 12-week scan
- NHS Best Start in Life, Hospital bag checklist
- ACOG, Committee Opinion 579: Definition of Term Pregnancy
- WHO, Recommendations on antenatal care for a positive pregnancy experience
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
How is the due date calculated in pregnancy?
Your due date is usually calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's rule. If you know your conception date, add 266 days instead. Your provider may then confirm or adjust the date with an early ultrasound.
Why is the due date calculated from the last period and not conception?
Most people know when their last period started, but few know the exact day they ovulated or conceived. Counting from the period gives everyone the same, easy starting point. It assumes ovulation about 14 days later, which is why the first two weeks of pregnancy happen before conception.
What is Naegele's rule?
Naegele's rule is a quick way to estimate a due date by hand: take the first day of your last period, add 7 days, count back 3 months and add 1 year. It gives the same result as adding 280 days, give or take a day because months have different lengths.
Does cycle length change my due date?
It can. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles are regularly longer, you probably ovulate later, so many calculators add the extra days. A 35-day cycle moves the estimate about a week later. An early scan settles it.
How accurate is my due date?
It's an estimate. The NHS says only about 1 in 20 babies arrive on their due date, and most are born between 37 and 42 weeks. Dating from a first-trimester scan is the most accurate method, so that's the date your midwife or doctor will usually use.
Related guides
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