Method
Using the last menstrual period, this calculator adds 280 days and adjusts by the selected cycle length. Using conception date, it adds 266 days. These are common estimating methods, not a diagnosis or confirmation of gestational age.
Example
If the last menstrual period started on 1 March 2026 and the cycle length is 28 days, the standard LMP estimate adds 280 days. If conception date is used instead, the calculator adds 266 days from that date.
Common mistakes
Do not treat the due date as a guaranteed birth date. Irregular cycles, uncertain ovulation date and ultrasound findings can change clinical dating.
How to interpret the result
The estimated due date is a planning date, not a guaranteed delivery date. Clinical dating may change after ultrasound findings or medical review.
Assumptions and limitations
Due dates are approximate. Cycle length, ovulation timing, ultrasound findings and clinical assessment can change dating. Contact a qualified healthcare professional for pregnancy care or medical concerns.
References
- NHS: dating pregnancy and scans, accessed 2026-05-13.
- ACOG: Methods for Estimating the Due Date, accessed 2026-05-13.
- MedlinePlus: When you pass your due date, accessed 2026-05-13.
Last reviewed: 2026-05-14