What is the most accurate method of dating a pregnancy?

Most Accurate Method of Dating a Pregnancy

The most accurate method of dating a pregnancy is a first trimester ultrasound when the crown-rump length (CRL) is at least 10mm. 1, 2

Accuracy of Different Dating Methods

First Trimester Ultrasound (Most Accurate)

• Crown-rump length (CRL) measurement in the first trimester provides the most accurate estimation of gestational age, accurate to within 3-5 days 3
• CRL has been demonstrated to have the lowest random and systematic errors compared to other biometric measurements 4
• First trimester dating via CRL is considered the gold standard against which other dating methods are compared 5
• The American College of Medical Genetics recommends CRL for pregnancy dating, noting it provides accurate gestational age estimation to within 7 days 2

Second Trimester Ultrasound

• Second trimester ultrasound dating based on multiple measurements (composite) is accurate to within 10 days 1
• For pregnancies at 14 weeks gestation or later, biparietal diameter (BPD) measurement can be used, but is less accurate than first trimester CRL 1
• Head circumference and corrected biparietal diameter are the most accurate predictors in the second trimester, but still less accurate than first trimester CRL 5

Sequential Third Trimester Ultrasounds

• The accuracy of all ultrasound predictors worsens progressively as pregnancy proceeds 5
• Third trimester measurements have the poorest accuracy for dating and should not be used when earlier measurements are available 5, 6

Last Menstrual Period (LMP)

• Although LMP dating is sufficiently accurate for screening purposes, gestational age estimation based on ultrasound measurements is more accurate 1
• LMP dating is subject to recall bias, irregular cycles, and delayed ovulation, making it less reliable than ultrasound dating 1

Clinical Algorithm for Pregnancy Dating

1. First priority: First trimester ultrasound with CRL ≥10mm (most accurate)
2. Second priority: Second trimester ultrasound before 24 weeks (if first trimester ultrasound unavailable)
3. Third priority: LMP when certain and with regular cycles (if no early ultrasound available)
4. Last resort: Sequential third trimester ultrasounds (least accurate)

Important Considerations

• Accurate pregnancy dating is critical for proper interpretation of screening results for conditions such as trisomies and neural tube defects 2
• If gestational age changes by 2+ weeks after ultrasound dating, screening results must be reinterpreted 2
• Gestational age is best expressed as weeks and days or decimal weeks (15 weeks and 5 days is 15.7 weeks) rather than rounded weeks 1
• For open neural tube defect screening, ultrasound dating based on BPD measurement can significantly improve detection of open spina bifida 1

Common Pitfalls to Avoid

• Do not rely on package insert (commercial) medians for gestational age dating, even for a short time 1
• Do not use third trimester measurements for dating when earlier measurements are available 5
• Do not round gestational age (15 weeks and 5 days should not be rounded to 16 weeks) 1
• Do not rely solely on LMP dating when ultrasound measurements are available, as ultrasound dating improves both sensitivity and specificity of screening 1

By following these evidence-based guidelines for pregnancy dating, clinicians can optimize prenatal care, improve screening accuracy, and ultimately reduce morbidity and mortality through appropriate timing of interventions.