Calculator guide
Estimate a pregnancy due date from the first day of the last menstrual period and cycle length.
Formula or method
Naegele-style estimate = LMP + 280 days, adjusted by cycle length − 28 days when an adjustment is entered.
How to use this calculator
- Enter the first day of the last menstrual period.
- Enter average cycle length if it differs from 28 days.
- Review the estimated date and trimester timeline.
Worked example
An LMP of January 1 with a 28-day cycle gives an estimated due date of October 8 (280 days later).
Understanding the result
A due date is an estimate; pregnancy length varies and clinicians may revise it using history or ultrasound.
Limitations and assumptions
- Irregular cycles, uncertain LMP, multiple pregnancy, and clinical findings can alter dating.
- It does not assess fetal health, labor timing, or pregnancy complications.
Frequently asked questions
Is the due date exact?
No. It is a planning estimate, and only a small share of births occur on that date.
What if I do not know my LMP?
A healthcare professional can discuss alternative dating methods.
When should I seek care?
Follow your clinician's prenatal guidance and seek urgent help for concerning symptoms.
Sources
Educational estimate only; not medical advice. Confirm dating with your healthcare provider.