How do you calculate gestational age according to the corrected Estimated Date of Delivery (EDD)?

Calculating Gestational Age from Corrected EDD

To calculate current gestational age from a corrected EDD, subtract 280 days (40 weeks) from the corrected EDD to determine the estimated date of conception, then count forward from that date to today's date to determine the current gestational age in weeks and days.

The Calculation Method

The calculation is straightforward once you understand that the EDD represents 280 days (40 weeks) from the last menstrual period:

• Subtract 280 days from the corrected EDD to determine the theoretical LMP date that corresponds to that EDD 1
• Count the number of days from that calculated LMP date to the current date to determine gestational age 1
• Express the result in weeks and days (e.g., 15 weeks 5 days) or decimal weeks (15.7 weeks) rather than rounded weeks for optimal accuracy 2, 1

Understanding "Corrected" EDD

The corrected EDD is typically based on the most accurate dating method available, which follows a specific hierarchy:

• First-trimester ultrasound crown-rump length (CRL) measurement between 8-13 weeks is the gold standard, providing accuracy within ±5-7 days and as precise as ±1.5 days compared to true gestational age 1
• When a discrepancy of ≥5 days exists between LMP and CRL dating in the first trimester, adopt the CRL-based gestational age for all clinical decision-making, which becomes your "corrected" EDD 1
• Second-trimester composite ultrasound dating at 18-22 weeks using multiple fetal measurements provides accuracy within ±10 days when first-trimester dating is unavailable 1

Special Circumstances

For assisted reproductive technology (ART) pregnancies, the calculation differs:

• Add 14 days to the number of completed weeks since fertilization to calculate gestational age, providing highly accurate dating since the exact fertilization date is known 2
• This method bypasses the need for EDD calculation since the conception date is precisely known 2

Critical Documentation Practices

Proper documentation ensures consistency in clinical care:

• Record both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date (corrected EDD) for all clinical decisions 1
• Never average LMP and ultrasound dates - always use the ultrasound date when available in the first trimester 1
• Express gestational age as weeks and days or decimal weeks throughout the pregnancy to maintain precision 2, 1

Common Pitfalls to Avoid

Several errors can compromise accuracy:

• Do not round gestational age to whole weeks - use completed weeks and days (12 weeks, 5 days is 12 completed weeks, not 13 weeks) 2
• Avoid using manual gestational age wheels, which have been shown to have significant bias with individual differences of up to 4 days; electronic devices are recommended 3
• Do not recalculate or change the EDD once established based on later ultrasounds, as accuracy decreases with advancing gestational age 1

Clinical Importance of Accurate Calculation

Precise gestational age calculation from the corrected EDD is essential for:

• Appropriate timing of Down syndrome and neural tube defect screening, as serum screening results are highly dependent on precise gestational age 1
• Maternal serum AFP levels increase by 10-15% per week in the second trimester, making accurate dating essential for interpretation 1
• Clinical decision-making regarding timing of delivery, particularly for post-term pregnancies and preterm labor management 4, 5