Your Due Date: How It's Worked Out
Finding out roughly when your baby is due is one of the first things most people want to know. Here is how the date is worked out, how accurate it is, and where to get your own estimate.
How the date is calculated
The standard method counts 280 days (40 weeks) from the first day of your last menstrual period — not from the date you think you conceived. Pregnancy is counted this way by convention, which is why you are considered about two weeks pregnant at the moment of conception.
An easy version of the same sum: take the first day of your last period, add seven days, then count back three months.
An important limitation
This calculation assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, the estimate will be off — sometimes by a week or more, because you will have ovulated earlier or later than the formula assumes.
If your periods are irregular, or you are unsure of the date of your last one, a calculator can only give you a rough idea.
A dating scan is more accurate
The most reliable way to date a pregnancy is an ultrasound dating scan, done between 8 and 12 weeks. It measures your baby directly rather than relying on the calendar, and in the first trimester it is accurate to within a few days.
If the scan date and the calculated date differ significantly, the scan date is normally used. See our guide to the dating scan.
How exact is a due date, really?
It is an estimate, not an appointment. Only about 5% of babies arrive on their due date. A birth any time from three weeks before to two weeks after is considered normal timing.
It is often more useful to think of a due month or a due fortnight than a due day — it makes the last few weeks considerably less frustrating.
What the date is used for
- Timing your antenatal appointments and scans
- Knowing when screening tests need to be done
- Monitoring your baby's growth against expectation
- Deciding what to do if labour starts early, or if the pregnancy goes past term
If you go past your due date
Many first pregnancies go a little beyond 40 weeks, which is normal. Monitoring usually increases after your due date, and if the pregnancy continues past around 41 to 42 weeks your doctor will discuss induction of labour with you. This is routine, and something to plan rather than worry about.
The bottom line
Your due date is 40 weeks from the first day of your last period, assuming a 28-day cycle. A dating scan between 8 and 12 weeks is more accurate and is what your care will be based on. Either way, treat it as a guide — only about 1 baby in 20 actually arrives on the day.
References
- National Health Service. Ultrasound scans in pregnancy [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/pregnancy/your-pregnancy-care/ultrasound-scans/
- National Health Service. Finding out you're pregnant [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/pregnancy/finding-out/