What is this calculator?
Estimate an expected due date from the first day of the last period.
What can you use it for?
Use it to understand the numerical relationship between the measurements or dates entered. Estimate an expected due date from the first day of the last period.
How to use it
- Enter first day of last menstrual period.
- Enter calculate as of.
- Select Calculate (or Generate for random tools).
- Read the answer and any additional results below the form. Check the units and the limits of the method.
Formula and method
Gestational age is the calendar difference from the last period to the selected calculation date.
Worked example
First day of last menstrual period: 2026-08-01, Calculate as of: 2026-10-08. The result is 9 weeks 5 days (estimated gestational age).
Assumptions and limits
LMP + 280 days assumes a typical 28-day cycle. Ultrasound and clinician dating may differ. Does not assess pregnancy health.
Frequently asked questions
What inputs does the Due Date Calculator need?
Use first day of last menstrual period, calculate as of. Read the answer and any additional results below the form. Check the units and the limits of the method.
What assumptions apply to the Due Date Calculator?
LMP + 280 days assumes a typical 28-day cycle. Ultrasound and clinician dating may differ. Does not assess pregnancy health.