Pregnancy Due Date Calculator

How the Pregnancy Due Date Calculator Works

This calculator estimates your due date, gestational age, and pregnancy timeline based on the first day of your last menstrual period (LMP). It uses Naegele’s Rule, the most widely used method for estimating due dates. These results are general estimates and do not replace professional prenatal care.

Formulas Used

The calculator uses the following standard rules:

• Naegele’s Rule (Due Date):
  LMP + 280 days (40 weeks)
• Cycle Length Adjustment:
  If cycle ≠ 28 days → Add (cycle − 28) days to due date
• Gestational Age:
  Today − LMP (in weeks + days)
• Trimester:
  • 1st Trimester: 0–13 weeks
  • 2nd Trimester: 14–27 weeks
  • 3rd Trimester: 28–40+ weeks

Example

Example for LMP = January 1, cycle = 28 days:

Due Date = Jan 1 + 280 days
         = October 8

If today is April 1:
Gestational Age = 13 weeks 6 days
Trimester = 2nd trimester