What is the expected date of delivery (EDD) if an ultrasound shows a gestational age of 6 weeks and 3 days as of September 30th, 2025?

Calculating Expected Date of Delivery (EDD) from Ultrasound Dating

The expected date of delivery (EDD) based on an ultrasound showing 6 weeks and 3 days on September 30th, 2025 would be May 19th, 2026.

Understanding Ultrasound Dating and EDD Calculation

• Ultrasound dating in early pregnancy (before 24 weeks) provides the most accurate estimation of gestational age, with accuracy within 5-14 days 1
• Crown-rump length (CRL) measurements at 11-13 weeks (or earlier) provide an accurate estimate of gestational age to within 7 days 1
• For accurate EDD calculation from ultrasound dating, gestational age should be expressed as weeks and days or decimal weeks rather than rounded weeks 1

Calculation Method

• Starting with September 30th, 2025 at 6 weeks and 3 days:
  • Full-term pregnancy is 40 weeks (280 days) from the first day of the last menstrual period 2
  • Remaining pregnancy duration = 40 weeks - 6 weeks 3 days = 33 weeks 4 days
  • Adding 33 weeks and 4 days to September 30th, 2025 gives an EDD of May 19th, 2026

Reliability of Ultrasound Dating

• Early ultrasound dating is more accurate than last menstrual period (LMP) recall for determining EDD 3
• First-trimester ultrasound dating has been shown to predict EDDs within 14 days of actual delivery date in approximately 88% of cases 3
• Electronic calculation methods are significantly more accurate than manual calculation methods, which can have errors of up to 4 days 4

Clinical Implications

• Accurate dating is critical for:
  • Planning appropriate timing for delivery 1
  • Interpreting screening tests that are gestational age-dependent 1
  • Identifying potential fetal growth restriction 1
  • Making decisions about antenatal corticosteroids and magnesium sulfate for neuroprotection if preterm delivery is anticipated 1

Common Pitfalls to Avoid

• Relying solely on LMP dating can lead to significant misclassification of preterm and post-term deliveries 3
• Manual calculation methods (wheels) have been shown to have poor accuracy with errors of up to 4 days 4
• Failing to document the method used for dating can lead to confusion in clinical management 5
• Not accounting for menstrual cycle length variations when using LMP dating can lead to significant errors in EDD calculation 5, 2

Best Practice Recommendations

• Document the ultrasound findings and dating method clearly in the medical record 1
• Use electronic calculation methods rather than manual wheels for greater accuracy 4
• For pregnancies dated by early ultrasound, use this as the definitive EDD rather than LMP-based calculations 3
• Ensure consistency in EDD documentation throughout pregnancy to avoid confusion in clinical management 5