Pregnancy Due Date Calculator
Estimate your baby’s due date from the first day of your last period or a conception date, with weeks-pregnant and date-window guidance.
Calculate your estimated due date
Choose the method that best matches what you know. The result is an estimate only — your midwife, GP or dating scan may give a different date.
How the pregnancy due date calculator works
The common last-period method estimates your due date by adding 280 days, or 40 weeks, to the first day of your last menstrual period. If your usual cycle is not 28 days, this calculator adjusts the estimate by the difference in cycle length.
Last period method:
estimated due date = first day of last period + 280 days + (cycle length - 28 days)
Conception method:
estimated due date = conception date + 266 daysNHS guidance says pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period, and if you do not know or are unsure of that date, you should speak to a midwife or GP.
Due date examples
How accurate is a due date calculator?
A due date calculator is useful for a first estimate, but it is not exact. Ovulation timing, irregular cycles, uncertain period dates, fertility treatment and early scan measurements can all change the estimate.
| Source of date | How to treat it |
|---|---|
| Last period date | Useful first estimate if your dates and cycle length are reasonably clear. |
| Conception date | Can be useful if you know it, but many people do not know the exact date. |
| Dating scan | NHS says the 10 to 14 week dating scan can give a more reliable due date. |
Use your maternity team’s date for appointments, screening windows and medical decisions. This calculator is only a planning tool.
What to do after you estimate your due date
- Use the date as a rough planning guide, not a fixed arrival day.
- Contact a GP or midwife if you have a positive pregnancy test and need to arrange care.
- Keep a note of your last period date, usual cycle length and any scan dates.
- Follow your midwife or GP’s advice if their estimated date differs from this calculator.
Pregnancy due date FAQs
Is my due date the day my baby will be born?
No. It is an estimated due date. Babies can arrive before or after that date, and your maternity team may update your date after a scan.
Why does the calculator use my last period?
Pregnancy dating is commonly counted from the first day of the last period, even though conception usually happens later. That is why a 40-week pregnancy estimate does not mean 40 weeks from conception.
What if I have irregular cycles?
The estimate may be less reliable. If your cycles are irregular, you are unsure of dates, or your scan gives a different date, follow your midwife or GP’s guidance.
Can this calculator tell me if my pregnancy is healthy?
No. It only estimates dates. It cannot assess symptoms, pregnancy risk, baby development or your health. Speak to a qualified health professional for care and advice.