How to use this calculator
Enter the first day of the last menstrual period and review the estimated gestational age and due date. This is a calendar estimate for orientation; use the due date established by your maternity care clinician for decisions.
Method and interpretation
The basic last-menstrual-period method adds 280 days, or 40 weeks, to the first day of the period. Gestational age is counted from that date rather than from a known conception date. The first trimester runs through 13 weeks and 6 days.
Assumptions and limits
The basic method assumes typical cycle timing and may differ when cycles are irregular or dates are uncertain. Ultrasound and clinical assessment can establish a different due date. A due date is not a prediction of the exact birth day, and the calculator cannot assess symptoms, fetal growth or pregnancy complications.
Worked example
If the last menstrual period began January 1, 2026, adding 280 calendar days gives an estimated due date of October 8, 2026. This example illustrates the date arithmetic, not an individualized clinical determination.