Last menstrual period method
The common LMP method estimates 40 weeks from the first day of the last menstrual period. Cycle length adjustments shift the estimate when the cycle is meaningfully longer or shorter than 28 days.
Conception-date method
When conception date is known, many calculators add about 266 days. This can still be uncertain because ovulation, fertilization and implantation timing are not always known precisely.
Limitations
- Ultrasound and clinical context can change dating.
- Cycle irregularity can make LMP estimates less reliable.
- A due date is not a guarantee of delivery date.
- Medical concerns should be discussed with a qualified clinician.
Due date methods and uncertainty
A common last-menstrual-period method estimates the due date as 280 days from the first day of the last period, assuming a 28-day cycle and ovulation around day 14. A 35-day cycle can shift the estimate because ovulation often occurs later than day 14.
Conception-date calculators usually add about 266 days from conception. Ultrasound dating can be more clinically useful in early pregnancy, especially when periods are irregular or the LMP date is uncertain. Public calculator results should be read the result as an estimate, not a medical scheduling decision.
Use the calculators
FAQ
Is the due date exact?
No. It is an estimate based on the chosen method and available information.
Why does cycle length matter?
Cycle length can shift the estimated ovulation timing used in LMP-based calculations.
Should I use the calculator instead of clinical dating?
No. Use clinical guidance for medical decisions and pregnancy care.
References
- ACOG: fetal growth during pregnancy, accessed 2026-05-16.
- NHS: due date calculator context, accessed 2026-05-16.
Last reviewed: 2026-05-16.