How to use Pregnancy Due Date Calculator

1. Prepare the required inputs

Estimate a pregnancy due date from the last menstrual period with a cycle-length adjustment. Use the units and options beside each field. Results update as inputs change; reset returns to example inputs.

First day of last period: calendar date
Usual cycle length (days): minimum 21, maximum 35

2. Try a worked example

Last period 1 January 2026 and a 28-day cycle gives an estimated due date of 8 October 2026.

3. Understand the calculation

Estimated due date = first day of last period + 280 days + (cycle length − 28). Gestational weeks use the adjusted dating origin.

4. Read the assumptions

For reasonably regular cycles. Ultrasound and clinical assessment can change dating. The 39-week date marks full term; 37 weeks is early term. This calculator does not confirm pregnancy.

5. Check and compare results

Match entered values to their labels and units. Review output labels alongside the assumptions, then change one input at a time. A currency selector changes display labels without converting amounts. Use the available save or copy actions and retain the input values so the estimate can be reproduced.

6. Fix an invalid input

Complete required fields and respect the listed bounds. Check date order, age relationships and mode-specific requirements where relevant; the error message identifies invalid combinations. Reset to example inputs to recover. A valid calculation applies to its stated model and does not verify a real-world outcome.