How do I calculate the expected date of delivery (EDD) from the last menstrual period (LMP)?

Calculating Expected Date of Delivery (EDD) from Last Menstrual Period (LMP)

To calculate the expected date of delivery (EDD) from the last menstrual period (LMP), add 283 days (40 weeks and 3 days) to the first day of the LMP. This is a modification of the traditional Naegele's rule which added 280 days.

Standard Method for EDD Calculation

1. Modified Naegele's Rule (Recommended):

   • Start with the first day of the last menstrual period (LMP)
   • Add 283 days (40 weeks + 3 days) to the LMP date 1
   • This provides the most accurate EDD calculation for women with regular cycles

2. Traditional Naegele's Rule (Less Accurate):

   • Add 280 days (40 weeks) to the first day of LMP
   • Research shows this method tends to predict EDD approximately 3.3 days too early 2

Step-by-Step Calculation

1. Identify the first day of the last menstrual period (LMP)
2. Add 7 days to the LMP date
3. Subtract 3 months from the LMP date
4. Add 1 year to the LMP date

Example:

• If LMP was June 10,2024
• Add 7 days → June 17,2024
• Subtract 3 months → March 17,2024
• Add 1 year → March 17,2025 (EDD)

Adjustments for Irregular Cycles

For women with irregular menstrual cycles:

• For cycles longer than 28 days: Add additional days equal to the difference between the cycle length and 28 days
• For cycles shorter than 28 days: Subtract days equal to the difference between 28 days and the cycle length

Example: For a 33-day cycle, add 5 extra days (33-28) to the calculated EDD 3

Accuracy Considerations

• The standard deviation for EDD prediction using LMP is approximately ±7 days 1
• Only 66.2% of EDDs calculated by LMP are within 14 days of the actual delivery date 4
• Electronic calculators are more accurate than manual calculation wheels, which can have errors of up to 4 days 5

Alternative Dating Methods

When LMP is uncertain or cycles are irregular, early ultrasound dating is preferred:

• Crown-rump length measurement between 7-14 weeks provides accuracy of ±7 days 1
• First-trimester ultrasound (ideally between 7-14 weeks) provides the most accurate dating 1
• Ultrasound dating and LMP-based dating (with the 283-day adjustment) have similar precision when LMP is known with certainty 2

Common Pitfalls to Avoid

• Using the traditional 280-day Naegele's rule (systematically predicts EDD too early)
• Failing to adjust for irregular menstrual cycles
• Using manual calculation wheels without validation (electronic calculators are more reliable) 5
• Relying solely on LMP when it is uncertain or when cycles are irregular

For standardized reporting, gestational age should be documented as the number of weeks plus the number of days (e.g., 12 weeks 3 days) 1.