Find your estimated due date instantly using Naegele’s Rule – the same method doctors use. Just enter the first day of your last menstrual period.
How the Due Date Is Calculated
This calculator uses Naegele’s Rule, the standard method used by doctors and midwives: add 280 days (40 weeks) to the first day of your last menstrual period. If your average cycle is longer or shorter than 28 days, the calculator adjusts the date accordingly.
Understanding Your Pregnancy Timeline
- Weeks 1-12 (First Trimester): Major organ development begins.
- Weeks 13-27 (Second Trimester): Often called the most comfortable stage; anatomy scan usually happens around week 20.
- Weeks 28-40 (Third Trimester): Rapid weight gain and final preparation for birth.
Only about 5% of babies are born exactly on their due date – most arrive within a 2-week window before or after.
Naegele’s Rule Step by Step
The traditional way to apply the rule by hand is:
If your last period began on 10 March 2026: add a year to get 10 March 2027, subtract three months to get 10 December 2026, then add seven days to get 17 December 2026. That is the same result as simply adding 280 days.
Adjusting for cycle length
Naegele’s Rule assumes a 28 day cycle with ovulation on day 14. Longer or shorter cycles shift ovulation, and therefore the due date, by the difference:
- A 35 day cycle pushes the due date 7 days later
- A 24 day cycle brings it 4 days earlier
This adjustment matters most for people with consistently long cycles, whose dates are otherwise systematically too early.
Why Ultrasound Dating Is More Accurate
LMP dating relies on two assumptions that are often wrong: that you remember the date precisely, and that you ovulated on day 14. Early ultrasound measures the embryo directly, and in the first trimester embryos grow at a very consistent rate.
| Method | Typical accuracy |
|---|---|
| Ultrasound, 8 to 13 weeks | Within about 5 to 7 days |
| Ultrasound, 14 to 20 weeks | Within about 7 to 10 days |
| LMP with a regular 28 day cycle | Within about 1 to 2 weeks |
| Ultrasound in the third trimester | Within about 3 weeks only |
Accuracy falls as pregnancy progresses, because babies grow at increasingly individual rates later on. This is why a dating scan done early is not repeated for dating purposes later, and why a due date set by an early scan is usually kept even if later scans differ.
Gestational Age vs Conception Age
Pregnancy is counted from the first day of your last period, not from conception. That means at the moment of conception you are already considered about 2 weeks pregnant.
- Gestational age: counted from LMP. This is what your provider, your notes and every pregnancy app use.
- Fetal or conception age: counted from ovulation, roughly 2 weeks less.
So a “40 week” pregnancy involves about 38 weeks of actual embryonic and fetal development.
Key Milestones
| Week | Milestone |
|---|---|
| 8 to 13 | Dating scan, most accurate confirmation of due date |
| 11 to 14 | Nuchal translucency and combined screening, where offered |
| 18 to 22 | Detailed anatomy scan |
| 24 | Generally regarded as the threshold of viability |
| 28 | Third trimester begins, appointments become more frequent |
| 37 | Early term begins |
What “Term” Actually Means
Obstetric guidance divides the end of pregnancy into named windows rather than treating 40 weeks as a single target:
| Label | Weeks |
|---|---|
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post term | 42 weeks 0 days and beyond |
Only around 5 percent of babies arrive on their due date, and roughly 90 percent are born within two weeks either side of it. A due date is best understood as the centre of a normal range rather than a deadline.
Frequently Asked Questions
How is my due date calculated?
Using Naegele’s Rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. By hand this is done as add one year, subtract three months, add seven days. If your last period started on 10 March 2026, the estimated due date is 17 December 2026.
How accurate is a due date calculator?
A due date from your last period is accurate to within about one to two weeks for most pregnancies with regular cycles. An ultrasound between 8 and 13 weeks is more accurate, usually within 5 to 7 days, because early embryos grow at a very consistent rate. Only about 5 percent of babies are born on their exact due date.
What if my cycle is not 28 days?
Naegele’s Rule assumes a 28 day cycle with ovulation on day 14, so a longer or shorter cycle shifts your due date by the difference. Add the difference for longer cycles and subtract it for shorter ones: a 35 day cycle moves the due date 7 days later, while a 24 day cycle moves it 4 days earlier.
What if I do not know my last period date?
An ultrasound dating scan is the most reliable alternative and is standard practice where the last period date is unknown or cycles are irregular. Booking that scan before 13 weeks gives the most accurate result, since dating accuracy falls as pregnancy progresses.
Is 40 weeks the same as 9 months?
Not quite. Forty weeks is about 9.2 calendar months, which is why pregnancy is commonly described as nine months. Counting also starts from the first day of your last period rather than from conception, so at the moment of conception you are already considered around 2 weeks pregnant and a 40 week pregnancy involves about 38 weeks of actual development.
Can my due date change during pregnancy?
Yes. Your provider may revise it after an early dating scan if that differs significantly from the estimate based on your last period. Once a due date has been set by an early scan it is usually kept, because dating from later scans is much less precise, within about three weeks in the third trimester.
What happens if I go past my due date?
It is common and usually not a cause for alarm. Pregnancy from 41 weeks is described as late term and from 42 weeks as post term. Providers typically increase monitoring after 40 weeks and will discuss induction, with the usual timing being somewhere between 41 and 42 weeks. Follow the guidance of your own midwife or doctor.