How to use the Pregnancy Due Date
- Enter the first day of your last menstrual period, or select conception dating.
- Adjust the cycle length if yours differs from 28 days.
- Read the estimated due date and current gestational age.
- Use a dating scan result in preference to this estimate if you have one.
How the calculation works
Naegele's rule adds 280 days — 40 weeks — to the first day of the last menstrual period, which assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, the estimate shifts by the difference, which is why the cycle length input matters. Dating from a known conception or embryo transfer date adds 266 days instead, since it skips the assumed two weeks to ovulation.
First-trimester ultrasound dating is more accurate than either method and is what clinicians use when the two disagree. Only about 4% of births occur on the estimated date and roughly 80% fall within two weeks either side, so the due date is best understood as the centre of a window rather than an appointment.
EDD = LMP + 280 days (Naegele's rule), adjusted by (cycle length − 28) ; or conception date + 266 daysSource: Naegele's rule; ACOG Committee Opinion on Methods for Estimating the Due Date (reaffirmed guidance recommends first-trimester ultrasound where available).
Worked example
A last menstrual period beginning 3 April 2026, with a regular 31-day cycle.
- Base estimate: 3 April + 280 days = 8 January 2027.
- Cycle adjustment: 31 − 28 = 3 days later.
- Adjusted EDD = 11 January 2027.
An estimated due date of 11 January 2027, with a normal-term window from about 28 December to 25 January.
Frequently asked questions
How accurate is a due date?+
Around 4% of babies arrive on the estimated date and about 80% within two weeks either side. Treat it as the middle of a range rather than a scheduled day.
What if my cycles are irregular?+
Then LMP dating is unreliable, because ovulation timing is unknown. A first-trimester ultrasound is the accurate option and is what your clinician will date from.
How is IVF dating different?+
The conception date is known precisely, so the estimate adds 266 days from a day-3 transfer date adjusted for embryo age, rather than assuming an ovulation day. Your clinic will provide the exact dating.
Should I rely on this instead of medical advice?+
No. This is an informational estimate. Antenatal care, dating scans and any decisions about your pregnancy should come from your midwife or doctor.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.