Pregnancy Due Date Calculator
Estimate your due date and current week.
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Find out roughly when your baby is due. Enter the first day of your last period (or your conception date) and this calculator estimates your due date, how many weeks pregnant you are today, and which trimester you're in.
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Due Date — The Quick Answer
Naegele's rule estimates the due date as the first day of your last period plus 280 days (40 weeks). Pregnancy is dated from the last period, not conception, which is about two weeks later.
Shortcut
Due date = first day of last period + 280 days (adjusted for cycle length)
Pregnancy milestones (from last period)
| Milestone | Week |
|---|---|
| End of 1st trimester | Week 13 |
| End of 2nd trimester | Week 27 |
| Full term begins | Week 37 |
| Due date | Week 40 |
| Post-term | Week 42+ |
The due date is an estimate, not a deadline. Only about 1 in 20 babies arrives on the exact date — most come within two weeks either side of it.
How It Works
The Method
Naegele's rule
Formula
Due date = LMP + 280 days + (cycle length − 28) From conception: due date = conception + 266 days
Variables
Last menstrual period
The first day of your last period. Pregnancy is conventionally dated from here.
40 weeks
The average length of pregnancy measured from the last period.
Adjustment
Longer or shorter cycles shift ovulation, so the due date is nudged by the difference from a standard 28-day cycle.
Note: Early ultrasound dating is more accurate than period-based calculations, especially for irregular cycles. Your maternity team will usually confirm or adjust your due date at a scan.
Worked Example: A Due Date from a 1 January Last Period
Naegele's rule with a standard 28-day cycle
Note the first day of the last period
1 January
Add 280 days (40 weeks)
1 January + 280 days ≈ 8 October
Adjust for cycle length
Longer than 28 days pushes it later; shorter brings it earlier
Work out the current week
Days since the last period ÷ 7 = weeks pregnant today
Reference Guide
| Milestone | Timing (from last period) |
|---|---|
| End of 1st trimester | Week 13 |
| End of 2nd trimester | Week 27 |
| Full term begins | Week 37 |
| Estimated due date | Week 40 |
| Post-term | Week 42 and beyond |
How to Read Your Due Date
An estimate, not an appointment
Only about 1 baby in 20 arrives on its exact due date. Most are born within two weeks either side, and anywhere from 37 to 42 weeks is considered full term. Treat the date as a helpful midpoint, not a deadline.
Best for: Planning around a window rather than a single day
Dating starts from the first day of your last period — about two weeks before conception — because that date is knowable while the exact day of conception usually isn't. This is why 'weeks pregnant' includes roughly two weeks before you actually conceived.
Best for: Making sense of the week count your clinic uses
Ultrasound dating in early pregnancy measures the baby directly and is more precise than a period-based estimate, especially for irregular cycles. If a scan disagrees meaningfully, your care team will usually adjust the date to the scan. This tool is an estimate, not medical advice.
Best for: Confirming dates with your midwife or doctor
How Is a Due Date Worked Out?
A due date estimates when a baby is likely to arrive, calculated by the long-standing Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. Pregnancy is dated from the last period rather than conception because that date can usually be identified, whereas the day of conception generally can't — which is why the 40-week count begins about two weeks before conception actually happens. This calculator also adjusts for cycle length and can work from a known conception date. The due date is an estimate: most babies arrive within two weeks either side, and an early ultrasound gives the most reliable dating.
Key Features
💡 Pro Tips
- →If your cycle isn't 28 days, enter your real cycle length — it meaningfully shifts the estimate for longer or shorter cycles.
- →Note the date your care provider confirms at your first scan; if it differs from this estimate, the scan date is the one to follow.
- →Use the week count to anticipate routine checks and milestones, but don't be alarmed if labour starts anywhere from week 37 to 42 — all of that is full term.
- →This tool is for planning and reassurance only; your midwife or doctor remains the source of truth for your pregnancy.
Common Mistakes
Treating the due date as a deadline
Only around 4–5% of babies are born on their due date. Expecting the exact day causes needless worry when it passes — a very common and usually unremarkable event.
Assuming the count starts at conception
Pregnancy is dated from the last period, about two weeks before conception. So at '6 weeks pregnant' the embryo is roughly four weeks old. Mixing the two up makes the weeks seem not to add up.
Ignoring cycle length with irregular cycles
Naegele's rule assumes a 28-day cycle. For longer, shorter, or irregular cycles the period-based estimate can be off, and an early ultrasound is far more reliable.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
American College of Obstetricians and Gynecologists (ACOG) (2017). Committee Opinion 700: Methods for Estimating the Due Date. Obstetrics & Gynecology.
Guidance on due-date estimation and the role of ultrasound dating.
View source - [2]
Jukic, A. M., Baird, D. D., et al. (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction, 28(10).
Evidence on the natural variation in pregnancy length around the due date.
View source - [3]
National Health Service (NHS) (2023). Working out your due date. NHS.uk.
Plain-language guidance on how due dates are calculated and used.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 8, 2026
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Frequently Asked Questions
How is a due date calculated?
Most commonly with Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This calculator also adjusts for your cycle length and can work from a known conception date instead.
Why is pregnancy dated from the last period?
Because the last period is a date most people can identify, whereas the exact day of conception usually isn't known. Dating from the last period means 'weeks pregnant' includes about two weeks before conception actually happened.
How accurate is the due date?
It's a good estimate but not exact — only around 4–5% of babies are born on their due date. Most arrive within two weeks before or after. Ultrasound dating in early pregnancy is more precise than period-based estimates.
What if my cycle isn't 28 days?
This calculator adjusts for that. A longer cycle means later ovulation, pushing the due date back; a shorter cycle brings it forward. For very irregular cycles, an ultrasound gives the most reliable date.