What is an estimated due date?
An estimated due date is the rough date a pregnancy may reach 40 weeks. It helps plan antenatal care, but it is not a guaranteed birth date.
An estimated due date, often shortened to EDD, is an approximate date for when a baby may be due. It is commonly worked out from the first day of the last menstrual period, adjusted for cycle length, or from an early pregnancy dating scan.
The key word is estimated. A due date helps organise antenatal appointments, scans and pregnancy planning, but many babies are not born on the exact date shown by a calculator.
Why an estimated due date matters
An estimated due date helps healthcare professionals work out roughly how far along a pregnancy is, when scans and screening tests may be offered, and whether a pregnancy appears earlier or later than expected.
It can also help with practical planning, but it should not be treated as a promise that labour will start on that exact day.
Estimate a pregnancy due date
Use the pregnancy due date calculator for a rough date based on the first day of the last period or a known conception date.
How an estimated due date is calculated
A common due date method adds 280 days, or 40 weeks, to the first day of the last menstrual period. If the menstrual cycle is longer or shorter than 28 days, some calculators adjust the result.
Estimated due date = first day of last period + 280 days
Adjusted estimate = result + (cycle length - 28 days)NHS guidance says its due date calculator works from the first day of the last period and that pregnancy normally lasts from 37 to 42 weeks from that date.
Worked example
Suppose the first day of the last period was 1 January 2026 and the cycle length is 28 days.
If the cycle length was 32 days instead, the calculator would add 4 extra days because 32 is 4 days longer than a typical 28-day cycle.
Why the due date can change
Estimated due dates can change because period dates may be uncertain, cycle lengths vary, ovulation may not happen on day 14, and early scan measurements may give a more reliable pregnancy age.
- Uncertain period date: if the first day of the last period is not known, a calculator result is weaker.
- Long or short cycles: ovulation timing can shift the estimate.
- Irregular periods: the last-period method may be less reliable.
- Early dating scan: NHS says the 10 to 14 week dating scan can work out a more reliable due date.
- Clinical context: your midwife, GP or maternity team may update the date based on your care record.
Due date calculator vs dating scan
A due date calculator is useful before scan information is available, but it is still only a rough estimate. NHS guidance says pregnant people in England are offered an ultrasound scan at around 10 to 14 weeks, called the dating scan, to see how far along the pregnancy is and work out a more reliable due date.
| Method | What it uses | How to treat it |
|---|---|---|
| Last period calculator | First day of last period and cycle length | Useful early estimate if dates are known. |
| Conception-date estimate | Known conception date plus around 266 days | Can help where conception date is clear, but still an estimate. |
| Dating scan | Ultrasound measurements in early pregnancy | Usually more reliable for antenatal planning. |
| Midwife or GP guidance | Your full clinical context | Should override a generic online calculator. |
Estimated due date FAQs
Is an estimated due date the day the baby will definitely be born?
No. It is a planning estimate, not a guaranteed birth date. NHS guidance says pregnancy normally lasts from 37 to 42 weeks from the first day of the last period.
What does EDD stand for?
EDD usually stands for estimated due date. Some people also use estimated date of delivery, but the important point is that the date is approximate.
Why did my due date change after a scan?
An early dating scan can estimate pregnancy age from ultrasound measurements. If that estimate differs from period-based dates, your maternity team may update the due date.
What if I do not know my last period date?
NHS guidance says to speak to a midwife or GP if you do not know the first day of your last period or are unsure.
Sources and notes
- NHS due date calculator: NHS — Due date calculator
- NHS dating scan guidance: NHS — 12-week scan
- Calculatorz note: this page is general information only and does not replace advice from a midwife, GP, obstetrician or maternity team.