Pregnancy Due Date Calculator
Estimate a due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). Ultrasound and clinical care often refine this estimate.
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Result
The formula
Naegele’s rule dates pregnancy from the first day of the last menstrual period and adds 280 days (40 weeks). That assumes a typical ~28-day cycle with ovulation near day 14. Known conception dating usually adds about 266 days from conception, and early ultrasound may revise the estimate when cycle history is uncertain.
Worked example
- LMP = 1 January 2026
- Add 280 days (40 weeks)
- Estimated due date ≈ 8 October 2026
Result: Estimated due date: 8 October 2026
How due dates are estimated
A due date is a midpoint estimate, not a promise. Most births occur in a window around that day.
Naegele’s rule (LMP dating)
Classic obstetric teaching adds one year, subtracts three months, and adds seven days to the LMP — equivalent to about 280 days for a regular 28-day cycle. This calculator uses the 280-day form.
Irregular cycles, recent hormonal contraception, or uncertain LMP reduce reliability. Tell your clinician if your cycle length differs from 28 days.
Conception and IVF dating
When conception date is known (including some fertility treatments), dating often uses about 266 days from conception (38 weeks). IVF protocols follow clinic-specific dating from egg retrieval or transfer — follow your clinic’s date, not a generic web estimate.
Ultrasound revisions
First-trimester ultrasound crown–rump length is often the preferred dating method when LMP is uncertain. Later scans assess growth relative to the established due date more than they freely reset it, unless there is a large discrepancy per local guidelines.
Term windows
Many guidelines describe early term, full term, late term, and post-term ranges rather than a single magic day. Only a small fraction of births occur exactly on the estimated due date.
Interesting facts
Named after Franz Naegele
German obstetrician Franz Karl Naegele popularized the LMP-plus-~280-days rule that still anchors many prenatal charts.
Only about 5% deliver on the day
Most births occur within about two weeks of the estimated due date. Midwife or obstetric follow-up refines timing with exams and ultrasound.
LMP vs conception dating
LMP dating assumes a typical cycle with ovulation around day 14. Known conception or embryo transfer dates change which day count applies.
Ultrasound adjustments
Early ultrasound dating is preferred when cycle history is uncertain; later scans primarily track growth against the agreed due date.
Full-term is a range
Clinical categories such as early term and full term emphasize a window of readiness rather than one calendar day.
Frequently asked questions
Add 280 days (40 weeks) to the first day of your last menstrual period. That is Naegele’s rule for a typical cycle. Your clinician may adjust after ultrasound.
It is an estimate. Dating scans often adjust the due date, especially with irregular cycles or uncertain LMP.
Conception-based dating usually adds about 266 days, but clinical protocols may still prefer LMP or ultrasound rules. IVF patients should use their clinic’s dating.
Obstetric “weeks pregnant” traditionally count from LMP, which is about two weeks before ovulation/conception in a 28-day cycle — so 40 weeks LMP ≈ 38 weeks from conception.
This tool uses the standard 280-day assumption. If your cycles are longer or shorter, ask your clinician how they adjust LMP dating.
References
- Methods for estimating the due date Clinical guidance on LMP vs ultrasound dating.
- Definition of term pregnancy Early-term, full-term, and related definitions.
- Antenatal care recommendations Global context for pregnancy care and timing discussions.