What is the expected date of delivery if the last menstrual period was May 16?

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

Based on a last menstrual period (LMP) of May 16,2025, the expected date of delivery (EDD) is February 21,2026.

Calculation Method

The standard method for calculating EDD from LMP follows these steps:

1. Start with the first day of the last menstrual period (May 16,2025)
2. Add 280 days (or 40 weeks) to determine the EDD

Important Considerations

• The traditional Naegele's rule adds 280 days to LMP, but evidence suggests adding 283 days may be more accurate 1
• Using LMP for dating is standard practice but has limitations:
  • Only sufficiently accurate when menstrual cycles are regular and 28 days 2
  • Less precise than ultrasound-based dating, particularly when cycle length varies from 28 days 3

Accuracy of LMP-Based Dating

• LMP-based dating can result in significant discrepancies:
  • On average, LMP dating predicts EDD to be approximately 3.3 days too early 1
  • Can lead to misclassification of preterm and post-term births 4
  • Only 66.2% of LMP-based EDDs fall within 14 days of actual delivery date, compared to 88.3% with early ultrasound 4

Improving EDD Accuracy

For more precise dating, especially in cases where:

• Menstrual cycles are irregular
• LMP is uncertain
• Clinical decisions are critical

Early ultrasound is recommended:

• First-trimester ultrasound, especially crown-rump length (CRL) measurement, provides the most accurate dating 2
• Ultrasound dating between 11-14 weeks is highly reliable 2
• Composite ultrasound measurements in second trimester are accurate to within 10 days 2

Clinical Implications

Accurate dating is crucial for:

• Optimal timing of prenatal screening tests 2
• Appropriate management of potential preterm or post-term pregnancies 4
• Reducing adverse events during delivery 5

When LMP and ultrasound dating differ significantly, ultrasound dating is generally preferred, particularly when the discrepancy exceeds 7-10 days in the first trimester 2.