How Pregnancy Due Dates Are Calculated

Most due date estimates begin with the first day of the last menstrual period (LMP). A standard estimate adds 280 days (40 weeks) to that date.

This approach is often called Naegele’s rule and assumes a regular cycle length. If cycles are longer or shorter, providers may adjust expected due dates.

Early ultrasound measurements can refine gestational age, especially when the LMP is uncertain. In clinical practice, ultrasound dating may be used to update the estimate.

A due date is a target window, not a guaranteed delivery day. Full-term births commonly happen within a range around the projected date.

Use online calculators for planning milestones, but always rely on your healthcare professional for medical guidance and final pregnancy dating decisions.