Due Date Calculator
Estimate your baby's due date using the NHS 40-week method. Enter the first day of your last period to see your due date, how many weeks pregnant you are, and your trimester.
Due date calculator
NHS 40-week (280-day) method.
An estimate only — not medical advice. Confirm with your midwife and the NHS pregnancy tools.
How the due date is worked out
The NHS and midwives date a pregnancy from the first day of your last menstrual period (LMP), not from conception. The standard estimate — known as Naegele's rule — adds 280 days (40 weeks) to that date. This calculator uses the same method, and adjusts the date if your cycle is longer or shorter than the typical 28 days.
Your estimated due date is 40 weeks (280 days) after the first day of your last period. Only about 1 in 20 babies arrives on the exact date, so think of it as the middle of a likely window rather than a fixed day.
Pregnancy trimesters
| Trimester | Weeks |
|---|---|
| First trimester | Weeks 1–12 |
| Second trimester | Weeks 13–27 |
| Third trimester | Weeks 28–40 |
Source: NHS — Working out your due date.
Why your dating scan matters
The LMP method assumes a regular 28-day cycle with ovulation around day 14. If your cycle is irregular, or you are not sure of your last period, your 12-week dating scan gives a more accurate due date by measuring the baby. Hospitals usually use the scan date for your maternity notes. To plan around ovulation instead, try our ovulation calculator; to work out maternity pay, see the statutory maternity pay calculator.
Frequently asked questions
How is a pregnancy due date calculated?
The standard NHS method counts 40 weeks (280 days) from the first day of your last menstrual period. This works because pregnancy is dated from that day, even though conception happens about two weeks later.
How accurate is the due date calculator?
It gives a good estimate, but only about 4% of babies arrive on their exact due date. Your dating scan at around 12 weeks gives a more precise date, especially if your cycle is irregular.
What if my cycle is not 28 days?
The 40-week method assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, your due date may shift by a few days. A dating scan adjusts for this.
How many weeks pregnant am I?
Your current week is the number of completed weeks since the first day of your last period. The calculator shows this plus the trimester you are in (1st: weeks 1-12, 2nd: 13-27, 3rd: 28-40).
Is this a substitute for medical advice?
No. This is an informational estimate only. Always confirm your dates and care with your midwife or GP, and use the NHS pregnancy due date tool for official guidance.