Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to estimate your due date.
| Method | Reference Point | Days Added |
|---|---|---|
| Last menstrual period (LMP) — Naegele's Rule | First day of last period | 280 days |
| Estimated conception date | Date of ovulation/conception | 266 days |
| Difference between the two methods | Typical 28-day cycle | 14 days |
FAQ
How is the due date calculated?
This calculator uses Naegele's Rule, the standard method: it adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle.
How accurate is this estimate?
It's an estimate, not a guarantee — only a small percentage of babies arrive exactly on their calculated due date. Actual delivery can happen anywhere from a couple of weeks before to two weeks after, and cycle length variations affect accuracy.
What if my cycle isn't 28 days?
This calculator assumes a standard 28-day cycle. If yours is significantly longer or shorter, your actual conception date — and due date — may differ; an ultrasound dating scan from a healthcare provider gives a more personalized estimate.
What are the trimesters?
Pregnancy is typically divided into three trimesters: first trimester (weeks 1-13), second trimester (weeks 14-27), and third trimester (weeks 28 to birth).
What's the difference between an LMP-based and a conception-based due date?
This calculator uses Naegele's Rule, which adds 280 days to the first day of your last menstrual period (LMP). A conception-based estimate instead adds about 266 days to the actual conception date — the two methods typically land about 14 days apart for a standard 28-day cycle.
Can I use this calculator if I know my exact conception date instead of my LMP?
This tool is built around the LMP method, so it's most accurate when you enter the first day of your last period. If you know your conception date instead, your due date is generally about 266 days after that date rather than 280.
What does 'full term' mean?
Full term is generally defined as weeks 39 through 40 of pregnancy. Babies born from week 37 are considered 'early term,' while babies born before week 37 are considered preterm.
What week of pregnancy am I in if I am 12 weeks and 3 days along?
You have completed 12 weeks, so you are in week 13 of pregnancy by the usual way of speaking. This calculator shows completed weeks, so it displays 'Week 12'.
How many days is 40 weeks?
40 weeks is 280 days. Adding 280 days to the first day of your last period gives the estimated due date used by this calculator.
When does the second trimester start?
In this calculator the 2nd trimester starts at week 13 and the 3rd at week 27. For a last period starting on 1 March 2026 that is May 31, 2026 and September 6, 2026. Some sources divide the trimesters slightly differently.
About this tool
This calculator estimates a pregnancy due date using Naegele's Rule, based on the first day of your last menstrual period, and shows your current estimated week and trimester. This is an estimate for general reference only, not medical advice — always confirm dates with a healthcare provider.
Worked example
If the first day of your last period was 1 March 2026, Naegele's Rule adds 280 days (40 weeks) to give an estimated due date of December 6, 2026. The calculator counts the number of whole weeks since that date, so after 12 weeks and 3 days it shows "Week 12". Weeks 0 to 12 are shown as the 1st trimester, weeks 13 to 26 as the 2nd, and week 27 onwards as the 3rd.
| Week | Date | Trimester in this calculator | Note |
|---|---|---|---|
| 0 | March 1, 2026 | 1st | Start of week 0 (first day of last period) |
| 4 | March 29, 2026 | 1st | Around the time a period is missed |
| 12 | May 24, 2026 | 1st | Last full week of the 1st trimester |
| 13 | May 31, 2026 | 2nd | 2nd trimester begins |
| 20 | July 19, 2026 | 2nd | About halfway |
| 27 | September 6, 2026 | 3rd | 3rd trimester begins |
| 37 | November 15, 2026 | 3rd | Early term begins |
| 40 | December 6, 2026 | 3rd | Estimated due date |
Why pregnancy is counted from the last period
Gestational age is counted from the first day of the last menstrual period, not from conception, because that date is usually known and conception is not. Conception normally happens about two weeks later, so by this method you are already "2 weeks pregnant" when conception occurs. That is why a pregnancy lasts 40 weeks by this count but about 38 weeks from conception.
If your cycle is not 28 days
Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation is later or earlier, and a clinician may adjust the due date by the difference. For the example above:
| Typical cycle | Adjustment | Due date for LMP of 1 March 2026 |
|---|---|---|
| 24 days | -4 days | December 2, 2026 |
| 26 days | -2 days | December 4, 2026 |
| 28 days | none | December 6, 2026 |
| 30 days | +2 days | December 8, 2026 |
| 32 days | +4 days | December 10, 2026 |
| 35 days | +7 days | December 13, 2026 |
This calculator uses the standard 280 days without any adjustment, so use the table above as a rough guide if your cycle is regular but different from 28 days.
A due date is an estimate
- Few babies arrive on the day. Only a small minority are born on their estimated due date, and many arrive in the weeks either side.
- "Full term" is a range. Clinical definitions generally class 37 weeks to 38 weeks 6 days as early term, 39 weeks to 40 weeks 6 days as full term, and 41 weeks to 41 weeks 6 days as late term.
- Ultrasound can change the date. A dating scan in the first trimester is more accurate than the last period for people with irregular cycles, and your healthcare provider may revise the date.
- Medical advice comes first. This tool is for general reference. Follow your midwife's or doctor's guidance for appointments and tests.