How is the due date worked out?
Pregnancy is counted from the first day of your last period, not from conception. Conception usually happens about two weeks later, so a full-term pregnancy lasts around 40 weeks (280 days) from your last period.
The traditional method is Naegele's rule: add 280 days to the first day of your last period (or add one year, take away three months and add seven days). This calculator does the same sum day by day.
Three ways to calculate
- Last period: due date = first day of last period + 280 days + (cycle length − 28). With a longer cycle you ovulate later, so the date moves later; with a shorter cycle it moves earlier.
- Conception date: due date = conception date + 266 days (38 weeks). For IVF, use transfer date + 263 days for a day-3 transfer or + 261 days for a day-5 transfer.
- Scan: due date = scan date + (280 − days of pregnancy measured). In the UK your due date is normally confirmed at the dating scan between 10 and 14 weeks, using the baby's crown–rump length, and this is the date the NHS uses.
Trimesters and key appointments
| Weeks | Stage | What usually happens |
|---|---|---|
| Up to 13 | First trimester | Booking appointment with your midwife; dating scan and combined screening at 11 to 13 weeks |
| 14 to 27 | Second trimester | 20-week (anomaly) scan between 18 and 20 weeks; glucose tolerance test at 24 to 28 weeks if you have risk factors |
| 28 to 40 | Third trimester | More frequent antenatal checks; from 37 weeks the baby is considered full term |
| 40 to 42 | After the due date | Membrane sweep and induction are usually offered; most units aim to induce before 42 weeks |
Worked example 1: from your last period
Last period 1 January 2026, 28-day cycle, checked on 1 May 2026
Due date = 1 January 2026 + 280 days = 8 October 2026
On 1 May, 120 days have passed = 17 weeks 1 day, in the second trimester
20-week scan window: 7 to 27 May 2026
Worked example 2: a longer cycle
Last period 1 January 2026, 35-day cycle
The cycle is 7 days longer than 28, so the due date moves 7 days later
Due date = 1 January 2026 + 280 + 7 days = 15 October 2026
Worked example 3: from a scan
Scan on 26 March 2026 measured 12 weeks 0 days
Days remaining = 280 − 84 = 196
Due date = 26 March 2026 + 196 days = 8 October 2026
Good to know
- Your due date is an estimate: most babies are born between 38 and 42 weeks.
- If your periods are irregular or you are unsure of the date of your last period, the dating scan gives a more reliable date.
- Maternity leave and Statutory Maternity Pay are based on your expected week of childbirth on the MATB1 form from your midwife or GP, so use that date for employer paperwork.
- Contact your midwife or maternity unit straight away if you have bleeding, severe pain or your baby's movements slow down or change, whatever your week of pregnancy.