What is an estimated due date (EDD)?
An Estimated Due Date, or EDD, marks the point at which a pregnancy is estimated to reach 40 weeks of gestation. It is a statistical midpoint: the average full-term pregnancy lasts around 280 days from the last menstrual period, but the range of what is considered full term spans from about 37 to 42 weeks. Research commonly cited in obstetric settings suggests roughly 4% of babies arrive on their exact calculated due date, while most arrive within a two-week window on either side.
You can get an instant estimate using our free due date calculator.
Method 1: Last menstrual period (LMP) — Naegele's rule
The most widely used baseline method is Naegele's rule. It works by taking the first day of the last menstrual period, adding 40 weeks (or equivalently, 280 days). The formula can also be stated as: add one year, subtract three months, add seven days.
This approach assumes a 28-day cycle with ovulation on day 14. For people with shorter, longer, or irregular cycles, the LMP-based estimate may be off by several days or more. It also requires an accurate recall of the LMP date, which is not always possible.
Method 2: Conception date — adding 266 days
If the date of conception is known with reasonable certainty — for example, following a single well-timed intercourse or insemination — it is possible to estimate the EDD by adding 266 days (38 weeks) to that date. This sidesteps the cycle-length assumption in the LMP method, but pinpointing a specific conception date is often uncertain.
Method 3: IVF transfer date
For pregnancies conceived through in-vitro fertilisation, the embryo transfer date provides an accurate and reliable anchor point for dating. The standard adjustments are:
- Day 5 blastocyst transfer: add 261 days to the transfer date.
- Day 3 embryo transfer: add 263 days to the transfer date.
IVF-dated pregnancies typically have the most precise EDD calculations because the fertilisation date is known. Healthcare providers generally accept IVF dating without the need for LMP-based revision.
Method 4: Ultrasound dating
An early ultrasound — particularly one performed between 8 and 14 weeks of pregnancy — can provide a highly accurate gestational age estimate by measuring the size of the embryo or foetus. Common measurements include crown-rump length (CRL) in the first trimester and biparietal diameter (BPD) or femur length in the second trimester.
When there is a meaningful discrepancy between the LMP-based estimate and the ultrasound measurement, many healthcare providers give preference to the ultrasound date, particularly if the scan was performed early in pregnancy. This is because early foetal growth is relatively consistent, making size a reliable indicator of gestational age at that stage.
Why estimates can differ
It is common for different calculation methods to produce slightly different results. The main reasons include:
- Cycle length variation — not everyone ovulates on day 14.
- Uncertainty about the LMP date.
- Natural variation in foetal size during the second and third trimesters.
- Multiple measurements at different scan appointments.
When results from different methods disagree, your healthcare provider or midwife will consider all available information to agree on a working EDD for your care.
What the due date means in practice
The EDD serves as an organisational anchor for prenatal care — scheduling scans, appointments, and planning. It is not a deadline. Most healthcare providers consider a birth full-term if it occurs between 37 and 42 weeks of gestation. Arriving a week or two before or after the EDD is entirely common and does not indicate anything has gone wrong.
Disclaimer
This guide is for informational and educational purposes only and is not medical advice, a diagnosis, or a substitute for professional care. Menstrual cycles, fertility, and pregnancy vary significantly between individuals. If you have concerns about your cycle, fertility, or pregnancy, consult a gynaecologist or healthcare provider.
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