Work out when your baby is due, from whichever date you actually have — your last period, a dating scan, a conception date or an IVF transfer. Shows the full term window as well as the date.
| Stage | Week | Date |
|---|
Your baby is due around 8 October 2026, but a pregnancy is term anywhere from 17 September 2026 to 22 October 2026 — so treat the date as the middle of a five-week window rather than a deadline.
Show your working
Breakdown
- Estimated due date
- 8 October 2026
- Term begins (37 weeks)
- 17 September 2026
- Overdue after (42 weeks)
- 22 October 2026
- Estimated conception
- 15 January 2026
Assumptions
- Naegele’s rule: 280 days from the first day of the last period, adjusted for cycle length.
- Ovulation is assumed 14 days after the last period, which is what the conception and IVF routes work back from.
- A dating scan is more accurate than a calendar date; where the two disagree, the scan should be followed.
- Term is 37 to 42 weeks. The due date is an estimate inside that window, not a deadline.
- Assumes a single pregnancy and a regular cycle. Not medical advice.
Due date facts
- How is a due date worked out?
- As 280 days — 40 weeks from the first day of your last period, assuming a 28-day cycle. This is Naegele’s rule.
- Why does it count from before I was pregnant?
- Because the last period is a date you can pin down and conception usually is not. It is why you are counted as two weeks pregnant at conception.
- What if my cycle is not 28 days?
- The date shifts by the difference. A 35-day cycle moves the due date seven days later.
- Will the baby arrive on that date?
- Almost certainly not. A pregnancy is term anywhere from 37 to 42 weeks — a five-week window around a single estimate.
Naegele’s rule
A due date is an estimate produced by a rule of thumb from the nineteenth century, and it has survived because it is simple and about as good as anything else: add 280 days to the first day of the last period. The older form of the same rule — add a year, take off three months, add seven days — gives the same answer.
The rule assumes a 28-day cycle with ovulation in the middle. If your cycle is longer, you ovulated later, the pregnancy is younger than the calendar suggests, and the due date moves out by the difference. This calculator makes that adjustment; many do not.
Which date should you use
A dating scan
The most reliable, and the one the NHS uses. A scan measures the baby rather than inferring from a calendar, so where a scan date and a period date disagree, the scan wins.
Your last period
The usual starting point before a scan. Reliable if your cycles are regular and you are sure of the date.
Conception date
Feels more precise but is not. It works backwards by assuming ovulation fourteen days after the period — the same assumption, applied in reverse.
IVF transfer
The exception. The embryo’s age at transfer is known exactly, so the dating is genuinely firmer than any of the others.
A date is not a deadline
Pregnancy is considered term from 37 weeks to 42 weeks. That is a five-week window, and the due date sits in the middle of it rather than at the end. Very few babies arrive on the day.
This is the reason this calculator shows the window beside the date. A single date invites people to treat it as a deadline, and it was never that precise.
Worked example
A last period beginning 1 January 2026 with a 28-day cycle. Adding 280 days gives an estimated due date of 8 October 2026. The term window runs from 17 September 2026 (37 weeks) to 22 October 2026 (42 weeks). With a 35-day cycle instead, the same last period gives 15 October 2026 — a week later.
Important notes
This is an estimate, not medical advice, and it does not replace a midwife or a dating scan. It assumes a single pregnancy and a regular cycle. If you are unsure of your dates, or your cycles are irregular, the figure here is a starting point for a conversation rather than an answer. Contact your midwife or GP with any concern about your pregnancy.
Evidence & Methodology
How This Page Is Grounded
Method
Applies Naegele’s rule — 280 days from the first day of the last menstrual period, adjusted for cycle length. A dating scan, conception date or IVF transfer date is first converted to an equivalent last-period date, so all four routes use the same arithmetic.
Important limitation: An estimate for a single pregnancy with a regular cycle, not medical advice. The conception and IVF routes assume ovulation fourteen days after the last period, so they are no more precise than the last-period route. A dating scan is more accurate than any calendar method.
Primary Sources
Quality Checks
All four dating routes, cycle-length adjustment, the 37 to 42 week term window and leap-year crossings are checked against worked examples derived from the published rule.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.