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Estimating a due date is one of the first things most expecting parents want to know. The standard method, Naegele's rule, estimates 280 days (40 weeks) from the first day of the last menstrual period — the same calculation most healthcare providers start with before any ultrasound dating.
This calculator also estimates conception date and trimester milestones from the same starting point.
How it works
Enter the first day of your last menstrual period. The tool adds 280 days for the estimated due date, 14 days for estimated conception, and standard trimester length markers for the end of the first and second trimesters.
- Enter first day of last menstrual period.
- Click Calculate to see your results.
Examples
Due date from a January period
A last period starting January 1 gives an estimated due date of October 8 of the same year.
Who should use it
- Getting an initial estimated due date early in pregnancy.
- Estimating trimester milestones for planning purposes.
Industry applications
- Prenatal care and healthcare
- Personal pregnancy planning
Advantages
- Instant estimate requiring only one date.
- Includes conception date and trimester milestones, not just the due date alone.
Limitations
- Assumes a standard 28-day cycle, which is less accurate for irregular cycles.
- Not a substitute for ultrasound-confirmed dating from a healthcare provider.
Common mistakes to avoid
- Treating the estimated due date as a guaranteed birth date rather than a general estimate.
- Using this calculation in place of an ultrasound-confirmed due date from a healthcare provider.
Best practices
- Use this as an initial estimate, then confirm dating with a healthcare provider, especially if your cycle is irregular.
Tips
- Bring this estimate to your first prenatal appointment — your provider will likely refine it with an ultrasound, especially if your cycle isn't a standard 28 days.