How to Calculate a Due Date
Hesaplayıcı2 min read
Short answer
The estimated due date is 280 days (40 weeks) after the first day of the last period. For a cycle longer than 28 days, add the difference; for a shorter one, take it away. From a known conception date, add 266 days; for IVF, add 261 or 263 days to the transfer day. This is general information, not medical advice.
Open the calculator →Formula
due date = first day of the last period + 280 + (cycle length − 28) days
- first day of the last period: the day the bleeding started
- cycle length: the days from the first day of one period to the first day of the next; 28 for many people
- 280 days: 40 weeks, the average length of a pregnancy
Conception is about 14 days after the first day of the last period. So:
- from conception:
due date = conception date + 266 days - for IVF:
due date = transfer day + 266 − embryo age days. Add 261 days for a day-5 embryo (blastocyst) and 263 days for a day-3 embryo. ACOG advises dating an IVF pregnancy from the transfer day.
Step-by-step example
For someone whose last period started on 1 January 2026, with a 28-day cycle:
- 1 January 2026 + 280 days = 8 October 2026. The NHS due date calculator gives the same date.
- With a 35-day cycle, 7 days are added: 15 October 2026.
- The estimated conception date is 266 days before the due date: 15 January 2026.
For a day-5 embryo transferred on 10 March 2026: 10 March 2026 + 261 days = 26 November 2026.
The due date calculator does these steps with your dates.
Pregnancy weeks and trimesters
Pregnancy weeks count from the first day of the last period; the first two weeks are before conception. On 1 June 2026, 151 days have passed since 1 January: 151 / 7 = 21 weeks 4 days. 129 days are left to the due date.
| Trimester | Weeks of pregnancy |
|---|---|
| 1st trimester | from the last period to 13 weeks 6 days |
| 2nd trimester | 14 weeks 0 days – 27 weeks 6 days |
| 3rd trimester | from 28 weeks 0 days to birth |
The limits are ACOG’s. 21 weeks 4 days is in the 2nd trimester.
When the ultrasound date wins
ACOG says a first-trimester ultrasound is the most accurate way to date a pregnancy. If the ultrasound and the last-period date differ by more than a set limit, the doctor changes the due date to the ultrasound one. After that, the due date changes only rarely. Most births come between weeks 37 and 42; few babies are born on the due date itself.
Worked example: Last period on 1 January 2026, a 28-day cycle: the week of pregnancy on 1 June 2026
Estimated due date
October 8, 2026
- Weeks pregnant
- 21 weeks
- Days after the full weeks
- 4 days
- Trimester
- 2
- Estimated conception date
- January 15, 2026
- Days to the due date
- 129 days
- Days pregnant
- 151 days
Step by step
days added = 280 + (cycle length − 28)
days added = 280 + (28 − 28) = 280 days
Days to the due date
days to the due date = due date − day of the calculation
days to the due date = 2026-10-08 − 2026-06-01 = 129 days
Days pregnant
days pregnant = 280 − days to the due date
days pregnant = 280 − 129 = 151 days
Weeks pregnant
weeks of pregnancy = ⌊days pregnant / 7⌋
weeks of pregnancy = ⌊151 / 7⌋ = 21 weeks
Days after the full weeks
extra days = days pregnant − 7 × weeks of pregnancy
extra days = 151 − 7 × 21 = 4 days
Trimester
trimester: under 14 weeks 1, under 28 weeks 2, later 3
trimester: 21 weeks = 2