Pregnancy Due Date Calculator
Work out an estimated due date from your last period, a known conception date, or an IVF transfer — with your current week and trimester dates.
- You are
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- Trimester
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- Days to go
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- Conceived around
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Key dates
| Milestone | Week | Date |
|---|
Choose a date to see your estimate.
How a due date is worked out
The standard method, Naegele's rule, counts 280 days — 40 weeks — from the first day of your last period. That deliberately starts about two weeks before conception actually happened, because the date of your last period is something you can know and the date of conception usually is not. It assumes a 28-day cycle with ovulation on day 14, so this calculator adjusts if your cycles are longer or shorter. Only about 4% of babies arrive on the estimated date; roughly 90% arrive within two weeks either side.
How accurate is a due date?
Less than the confident single date suggests. The due date is the midpoint of a distribution, not a prediction.
| Arrival | Roughly |
|---|---|
| On the estimated due date | 4% |
| Within 1 week either side | ~70% |
| Within 2 weeks either side | ~90% |
| Before 37 weeks (preterm) | ~10% |
| After 42 weeks | Under 1% with modern monitoring |
First pregnancies tend to run slightly longer than subsequent ones. An early dating scan, usually between 8 and 14 weeks, measures the baby directly and is more accurate than any date-based calculation — if it disagrees with this by more than about five days, the scan date is the one clinicians use.
Why cycle length matters
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are consistently 35 days, you likely ovulate around day 21 — a week later — and the standard calculation would place your due date a week early.
This calculator shifts the due date by the difference between your cycle length and 28 days. With a 35-day cycle the due date moves seven days later; with a 24-day cycle it moves four days earlier.
If your cycles are irregular, date-based estimates are unreliable and a dating scan matters considerably more.
IVF dates are the precise ones
IVF is the one case where conception timing is known exactly, which makes these due dates the most reliable of all.
The calculation works backwards from the transfer: a day-5 blastocyst is already five days past fertilisation, and fertilisation is treated as day 14 of the notional cycle. So the notional last period is 19 days before a day-5 transfer, and 17 days before a day-3 transfer.
How to use the Pregnancy Due Date Calculator
- Choose what you are calculating from — your last period is the usual starting point.
- Enter the date. For a last period, this is the first day of bleeding.
- Adjust your average cycle length if it is not 28 days.
- Read your due date, current week, and the key dates table.
Frequently asked questions
How many weeks pregnant am I?
Enter the first day of your last period and the calculator shows your current week and day. Pregnancy is counted from that date, so you are considered about two weeks pregnant at conception.
Why is pregnancy dated from my last period?
Because it is a date you can actually identify. Conception typically occurs around two weeks later, which is why 40 weeks of pregnancy is really about 38 weeks of foetal development.
What if my cycle is not 28 days?
Enter your average length and the due date is adjusted. Longer cycles mean later ovulation and a later due date.
Which is more accurate, this or my scan?
The scan. An early dating scan measures the baby directly, and clinicians will re-date a pregnancy if the scan differs from the period-based estimate by more than about five days.
When does each trimester start and end?
First trimester is weeks 1–12, second is 13–27, third is 28 to birth. The exact boundaries vary slightly between sources, and the key dates table above shows them for your pregnancy.
Related tools
Last reviewed September 4, 2026. An estimate only. Your midwife or doctor’s dating scan is more accurate and takes precedence.