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Due Date Calculator
Your last period
Weeks pregnant and the countdown are counted up to today. This calculator gives an estimate for information only and is not medical advice — only an ultrasound can confirm your due date. If anything worries you, talk to your doctor or midwife.
Example: first day of last period August 3, 2026, with a 28-day cycle → due May 10, 2027, conception around August 17, 2026.
Behind the numbers
How your due date is calculated
Doctors count pregnancy as 40 weeks from the first day of your last period, even though conception usually happens about two weeks later. If your cycle is longer or shorter than 28 days, ovulation shifts by the same amount, so the due date shifts with it.
Conception ≈ last period + 14 days + (cycle − 28). For the example: August 3, 2026 + 280 days = May 10, 2027, and conception was around August 17, 2026.
The three trimesters
| Trimester | Weeks (from last period) | Dates for the example |
|---|---|---|
| 1st trimester | 0 – 13 weeks, 6 days | Aug 3 – Nov 8, 2026 |
| 2nd trimester | 14 weeks – 27 weeks, 6 days | Nov 9, 2026 – Feb 14, 2027 |
| 3rd trimester | 28 weeks – due date | Feb 15 – May 10, 2027 |
Weeks pregnant = (today − first day of last period) ÷ 7. For the example, on November 9, 2026 you would be 14 weeks, 0 days pregnant, in the second trimester.
A few comparisons
Example due dates
| Last period | Cycle | Conception date | Due date |
|---|---|---|---|
| Aug 3, 2026 | 28 days | Aug 17, 2026 | May 10, 2027 |
| Aug 17, 2026 | 28 days | Aug 31, 2026 | May 24, 2027 |
| Mar 2, 2026 | 35 days | Mar 23, 2026 | Dec 14, 2026 |
| Jun 8, 2026 | 24 days | Jun 18, 2026 | Mar 11, 2027 |
Getting the most from your due date.
Write down the first day of your last period as soon as you get a positive test — it is the date every due date calculation starts from, and it is easy to forget later.
Book an early ultrasound if you can: a first-trimester scan measures the baby and usually gives a more exact due date than the last-period calculation, especially if your cycles are irregular.
If your cycle is not 28 days, set your real average cycle length above — a 35-day cycle moves the due date a full week later than a 28-day cycle with the same last period.
Treat the due date as the middle of a window, not a deadline: most babies arrive within two weeks either side of it, so have your hospital bag ready from about 37 weeks.
Good to know
Your due date questions, answered.
How accurate is a due date from my last period?
It is an estimate, not a promise: only a small share of babies arrive on their exact due date, and most arrive within two weeks either side of it. An ultrasound in the first trimester gives a more exact date, because it measures the baby directly.
Why is the due date 280 days after my last period?
Pregnancy is counted as 40 weeks from the first day of your last period, which is about 38 weeks from conception. The two extra weeks cover the time before ovulation, when you were not yet pregnant.
What if my cycle is longer or shorter than 28 days?
Ovulation usually happens about 14 days before your next period, so a longer cycle pushes ovulation — and the due date — later. This calculator shifts the date by the difference from 28 days: a 35-day cycle moves the due date 7 days later than a 28-day cycle with the same last period.
What if I do not know the first day of my last period?
Use the date of your earliest ultrasound instead: your doctor or midwife can work out the due date from the baby's measurements. If you conceived by IVF, try our IVF due date calculator, which counts from the transfer date.