How the calculation works
Pregnancy is counted in weeks from the first day of the last period, which is about two weeks before conception. A full pregnancy is 40 weeks, or 280 days.
Due date = first day of last period + 280 days + (cycle length − 28 days)
Due date from conception = conception date + 266 days
Weeks pregnant = whole weeks since the start (due date − 280 days), with the days that are left over.Worked example
Last period began on 10 January 2026, 28-day cycle, progress on 20 June 2026.
- Due date: 10 January 2026 + 280 days = 17 October 2026, a Saturday.
- From 10 January to 20 June is 161 days, which is 23 weeks and 0 days. That is the second trimester.
- The second trimester began 18 April 2026, the halfway point of 20 weeks was 30 May 2026, and 37 weeks is reached on 26 September 2026.
With a 32-day cycle the due date moves 4 days later, to 21 October 2026. A conception date of 24 January 2026 gives 17 October 2026.
What this calculator does not include
- An ultrasound dating, which your midwife or doctor may use instead of this estimate.
- Irregular cycles, twins or other multiple births, and IVF dating, which follow other rules.
- Medical advice. Ask your midwife or doctor about your pregnancy.
Results are estimates for information, not medical advice. Last updated: 12 November 2026.
Frequently asked questions
How is the due date worked out?
The most common method adds 280 days, which is 40 weeks, to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves by the same number of days. From a conception date, the calculator adds 266 days.
How accurate is a due date?
It is an estimate. Only a small share of babies, around 4 or 5 in 100, are born on the due date. Most are born between 37 and 42 weeks.
Which date does my midwife or doctor use?
Your midwife or doctor sets the date that counts, often from an early ultrasound. If their date differs from the one here, follow theirs.
What if my cycle is irregular or I do not know the date of my last period?
Then this estimate may be off by a week or more. An early ultrasound gives a better date, so ask your midwife or doctor.
What do the trimesters mean here?
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. Pregnancy age is counted from the first day of the last period, so you count as 4 weeks pregnant at about the time a period is missed.