How to use
- Choose what you know: your last period, the conception date or an IVF transfer date.
- Enter the date. For periods, add your usual cycle length if it isn’t 28 days.
- See your due date, how many weeks pregnant you are today, your trimester and key dates.
How the due date is calculated
- Last period: first day of your last period + 280 days (40 weeks), the convention ACOG uses.
- Cycle length: the date moves by your cycle length minus 28 days, as the NHS calculator does.
- Conception: conception date + 266 days.
- IVF: day-5 embryo transfer + 261 days; day-3 transfer + 263 days (ACOG).
Weeks of pregnancy count from the first day of your last period, so conception happens at about 2 weeks.
Examples
- Last period 1 May 2026, 28-day cycle: due Friday, 5 February 2027. On 21 October 2026 that is 24 weeks 5 days (24+5), in the second trimester.
- Same date, 35-day cycle: due 12 February 2027, 7 days later.
- Day-5 IVF transfer on 20 May 2026: due 5 February 2027.
Trimesters
| Trimester | US (ACOG) | NHS (UK) |
|---|---|---|
| First | Up to 13+6 | Up to week 12 |
| Second | 14+0 to 27+6 | Weeks 13 to 27 |
| Third | 28+0 onward | Week 28 onward |
Term (ACOG)
| Label | Weeks |
|---|---|
| Early term | 37+0 to 38+6 |
| Full term | 39+0 to 40+6 |
| Late term | 41+0 to 41+6 |
| Post-term | 42+0 and later |
How accurate is it?
Only about 4 in 100 babies arrive on their due date. Birth any time from 37 to 42 weeks is normal. An early dating scan, around 10 to 14 weeks, is the most accurate way to set the date.
Naegele's rule and the calendar shortcut
The shortcut “add a year, subtract 3 months, add 7 days” can land up to 3 days later than 280 days. For a last period on 1 May 2026 it gives 8 February 2027 instead of 5 February.
We always add the exact number of days.
This is an estimate, not a diagnosis. Your midwife or doctor will confirm your dates. Planning ahead? The water intake calculator shows how much to drink during pregnancy.