Pregnancy
How To Calculate A Due Date
A due date is forty weeks counted from the first day of the last menstrual period, and the arithmetic is a single addition of 280 days.
Quick Answer
due date = last period + 280 days
- LMP
- First day of the last menstrual period
- 280
- Days in forty weeks, the length of a typical pregnancy from the LMP
- due date
- LMP plus 280 days, the estimated date of delivery
Count forty weeks, which is 280 days, forward from the first day of the last menstrual period. A last period beginning 10 February 2026 gives a due date of 17 November 2026. The method assumes a regular 28 day cycle, so ovulation around day 14.
What Is A Due Date?
A due date is an estimate of when a pregnancy will reach forty weeks, counted from the first day of the last menstrual period rather than from conception. That starting point is used because the last period is a date most people can recall, while conception usually cannot be pinned down to a single day.
Naegele's rule is the arithmetic behind the estimate: add 280 days, which is exactly forty weeks, to the first day of the last period. A common shortcut is to subtract three months and add a year, then add seven days, which gives the same answer for most dates without counting weeks.
Counting from the last period means the estimate starts roughly two weeks before conception. At the moment of conception the pregnancy is already considered two weeks along. That offset surprises people, but it is consistent with how every clinical milestone is measured.
The 280 day figure assumes a regular 28 day cycle with ovulation on day 14. A longer cycle shifts ovulation later and therefore shifts the due date later, by roughly one day for each day the cycle exceeds 28. A shorter cycle shifts it earlier. The adjustment is small but real for anyone with a consistently irregular cycle.
Conception, when it can be identified, is taken as about fourteen days after the last period began. That places it around 24 February 2026 for a last period beginning on 10 February, and the due date sits 266 days after that point rather than 280.
The estimate is a midpoint, not a deadline. Only a small minority of babies arrive exactly on the due date, and a pregnancy is considered full term from 37 weeks through 42 weeks. That window is about five weeks wide, and it is the reason the due date is described as an estimate rather than a date.
The trimester boundaries are counted from the same starting point. The first trimester runs to the end of week 13, the second through the end of week 27, and the third from week 28 to birth. For a last period beginning 10 February 2026, the first trimester ends on 12 May 2026 and the second on 18 August 2026.
Ultrasound dating often revises the estimate. A scan in the first trimester measures the baby's crown-rump length and can date the pregnancy more precisely than the last period, particularly where the cycle is irregular. When a scan and Naegele's rule disagree, clinicians normally follow the scan.
Gestational age is simply the number of days since the last period, expressed in weeks and days. It is not affected by when the pregnancy was confirmed or when symptoms began. Everything from screening windows to maternity leave eligibility is keyed to this count.
This page deliberately reports only dates derived from the last period and never reads the current date, because the calculator is served as a static page that would otherwise freeze at the moment it was built. To find how far along you are today, count the days from the last period to today and divide by seven.
Formula
due date = LMP + 280 days
Forty weeks counted from the first day of the last menstrual period. The most common alternative is to subtract three months, add a year and add seven days.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| LMP | Last menstrual period | date | The first day of bleeding, not the last. |
| 280 | Days | days | Forty weeks, the conventional length of a pregnancy from the LMP. |
| D | Due date | date | The estimated date of delivery. |
conception = LMP + 14 days
Assumes ovulation on day 14 of a 28 day cycle. Conception therefore falls about two weeks after the count begins.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| LMP | Last menstrual period | date | First day of bleeding. |
| 14 | Days | days | Typical ovulation day in a 28 day cycle. |
| C | Conception | date | Estimated fertilisation date. |
gestational age = days since LMP / 7
Weeks of pregnancy, counted from the last period rather than from conception. This is the figure every clinical milestone uses.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| days | Days elapsed | days | Days from the last period to the date in question. |
| g | Gestational age | weeks | Pregnancy duration in weeks. |
How To Calculate A Due Date
- 1
Identify the first day of the last period
The count begins on the first day of bleeding, not the last day. Using the final day instead shifts the whole estimate by the length of the period, typically three to five days.
- 2
Add 280 days
Forty weeks is 280 days. Counting forward from 10 February 2026 lands on 17 November 2026. Adding 40 weeks to a calendar date reaches the same place, since a week is seven days.
- 3
Adjust for cycle length if it is not 28 days
Add one day for each day the usual cycle runs beyond 28, and subtract one for each day it runs short. A consistent 32 day cycle moves the estimate about four days later.
- 4
Work out the trimester boundaries
The first trimester ends at the close of week 13 and the second at the close of week 27. For a last period beginning 10 February 2026 those fall on 12 May 2026 and 18 August 2026 respectively.
- 5
Treat the result as a window, not a deadline
Full term spans 37 to 42 weeks, a window of roughly five weeks. The due date is the centre of that range, and a first-trimester ultrasound will often refine it further.
Examples
Example 1: Last period beginning 10 February 2026
- First day of last period
- 10 February 2026
- Typical cycle length
- 28 days
| Step | Calculation | Result |
|---|---|---|
| Estimated conception | 10 February 2026 + 14 days | 24 February 2026 |
| End of first trimester | 10 February 2026 + 13 weeks | 12 May 2026 |
| End of second trimester | 10 February 2026 + 27 weeks | 18 August 2026 |
| Estimated due date | 10 February 2026 + 280 days | 17 November 2026 |
Result: A last period beginning 10 February 2026 gives an estimated due date of 17 November 2026, with conception around 24 February 2026.
Example 2: Last period beginning 1 June 2026
- First day of last period
- 1 June 2026
- Typical cycle length
- 28 days
| Step | Calculation | Result |
|---|---|---|
| Estimated conception | 1 June 2026 + 14 days | 15 June 2026 |
| Estimated due date | 1 June 2026 + 280 days | 8 March 2027 |
Result: A last period starting 1 June 2026 puts the estimated due date at 8 March 2027, forty weeks later.
Example 3: Last period beginning 20 September 2026
- First day of last period
- 20 September 2026
- Typical cycle length
- 28 days
| Step | Calculation | Result |
|---|---|---|
| Estimated conception | 20 September 2026 + 14 days | 4 October 2026 |
| Estimated due date | 20 September 2026 + 280 days | 27 June 2027 |
Result: A last period beginning 20 September 2026 gives a due date of 27 June 2027, forty weeks later.
Calculator
Prefer a full-width tool? Open the A Due Date calculator page.
Common Mistakes
Counting from conception instead of the last period
Clinical dating begins on the first day of the last period, which is roughly two weeks before conception. Starting from conception makes the pregnancy appear two weeks shorter than every chart and screening window expects.
Using the last day of the period rather than the first
The count begins on the first day of bleeding. Using the final day shifts the estimate later by the length of the period, which is usually three to five days and can move a borderline screening window.
Treating the due date as a delivery date
Full term spans 37 to 42 weeks, so the due date is the middle of a five-week window rather than a fixed appointment. Most babies arrive on either side of it.
Ignoring a cycle that is not 28 days
The 280 day rule assumes ovulation on day 14. A consistently longer or shorter cycle shifts the estimate, and the adjustment is about one day for each day the cycle differs from 28.
Assuming a 30 day month makes the arithmetic easy
Forty weeks is 280 days, not nine months of thirty days each, which would be 270. Counting in whole months rather than weeks accumulates an error of about ten days.
Discounting a first-trimester ultrasound
Scan dating in the first trimester is generally more accurate than the last-period rule, especially with irregular cycles. When the two disagree, the scan is normally the one that is followed.
FAQ
How is a due date calculated?
Add 280 days, which is forty weeks, to the first day of the last menstrual period. A last period beginning 10 February 2026 gives a due date of 17 November 2026. This is Naegele's rule, and it assumes a regular 28 day cycle.
Why is pregnancy counted from the last period and not conception?
The first day of the last period is a date most people can recall accurately, whereas conception usually cannot be pinned to a single day. Counting from the last period also matches how every clinical milestone and screening window is defined.
What if my cycle is longer or shorter than 28 days?
Adjust the estimate by about one day for each day your cycle differs from 28. A consistent 32 day cycle moves the due date roughly four days later, since ovulation happens later in a longer cycle.
Is the due date exact?
No. It is an estimate with a window of about five weeks, since full term runs from 37 to 42 weeks. Only a small proportion of babies arrive on the due date itself.
When do the trimesters start and end?
The first trimester runs to the end of week 13, the second through the end of week 27, and the third from week 28 until birth. All three are counted from the first day of the last period.
Can an ultrasound change my due date?
Yes. A first-trimester scan measures the baby's crown-rump length and can date the pregnancy more precisely than the last-period rule, particularly when cycles are irregular. Where the two disagree, the scan usually takes precedence.
References
- [1]Wikipedia, Naegele's rule — https://en.wikipedia.org/wiki/Naegele%27s_rule
- [2]Wikipedia, Gestational age — https://en.wikipedia.org/wiki/Gestational_age
- [3]American College of Obstetricians and Gynecologists, Methods for estimating the due date — https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
- [4]World Health Organization, Prenatal care — https://www.who.int/health-topics/maternal-health