How to use
- Choose what you know, such as the first day of your last period.
- Enter the date and, for periods, your usual cycle length.
- Read your due date, current week and trimester.
- Check the key dates, such as the anatomy scan and full term.
Options explained
- Last period
- The standard method used by doctors. The due date is 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is longer or shorter than 28 days.
- Conception and IVF
- Conception is counted as two weeks after the start of the period. For IVF, the embryo's age at transfer (day 3 or day 5) is taken into account.
- Ultrasound
- If a scan dated your pregnancy, enter the scan date and the weeks and days it showed. Early scans are the most accurate way to date a pregnancy.
Worked example and key dates
If the first day of your last period was 10 January 2026 and your cycle is 28 days, the due date is 280 days later: 17 October 2026. On 18 April 2026 you are 14 weeks 0 days pregnant, in the second trimester. With a 32-day cycle the date moves by the 4 extra days, to 21 October 2026.
The other methods are converted to the same starting point, an equivalent first day of the last period:
last period: the date, plus (cycle length − 28) days
conception: conception date − 14 days
IVF transfer: transfer date − 14 days − embryo age (3 or 5 days)
ultrasound: scan date − (weeks × 7 + days) found at the scan| Milestone | Week | Date in the example |
|---|---|---|
| End of first trimester | 13 weeks 6 days | 17 April 2026 |
| Anatomy scan | 20 weeks | 30 May 2026 |
| Viability | 24 weeks | 27 June 2026 |
| Third trimester begins | 28 weeks | 25 July 2026 |
| Full term | 37 weeks | 26 September 2026 |
| Due date | 40 weeks | 17 October 2026 |
The week counting follows ACOG Committee Opinion 700.
Frequently asked questions
How accurate is the due date?
It is an estimate. Only about 4% of babies are born on their due date; most arrive within two weeks either side of it.
Why does pregnancy count from my last period?
The exact day of conception is rarely known, while the start of the last period usually is. Counting from it adds about two weeks before conception, which is why a pregnancy lasts 40 weeks on paper.
What are the trimesters?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.
Is my information stored?
No. The calculation happens in your browser and nothing is saved or sent.
Privacy
This tool runs entirely in your browser. Your files and text are processed on your device and are never uploaded to our servers.