When is a dating ultrasound typically performed during pregnancy?

Dating Ultrasound Timing in Pregnancy

A first trimester ultrasound with crown-rump length measurement, ideally performed between 10-13 weeks gestation, is the most accurate method for pregnancy dating and should be the standard of care for all pregnancies. 1

Optimal Timing for Dating Ultrasound

First Trimester (Preferred)

• 10-13+6 weeks: Ideal timing for dating ultrasound
  • Crown-rump length (CRL) measurement provides accuracy within ±7 days 1
  • Most accurate when performed earlier rather than later in pregnancy 2
  • First trimester dating is superior to Last Menstrual Period (LMP) in predicting delivery date by at least 1.7 days 3
  • Reduces post-term pregnancy diagnoses from 10.3% to 2.7% 3

Second Trimester

• 14-20 weeks: If first trimester dating was missed
  • Composite measurements (multiple parameters) are accurate to within 10 days 4
  • Biparietal diameter (BPD) measurement can be used after 14 weeks 4, 1
  • Less accurate than first trimester CRL measurements 2
  • After 24 weeks, a reliable LMP may provide better estimates than ultrasound 2

Measurements Used for Dating

1. First Trimester:

   • Crown-rump length (CRL) ≥10mm is the gold standard 1
   • Gestational sac and yolk sac visualization (around 5-5.5 weeks) 4
   • Embryo with cardiac activity typically visualized at 6 weeks 4

2. Second Trimester:

   • Biparietal diameter (BPD) - can be used after 14 weeks 4
   • Head circumference (HC) 5
   • Femur length (FL) 5
   • Abdominal circumference (AC) 1
   • Combination of measurements (composite) improves accuracy 4

Hierarchy of Dating Methods

The recommended hierarchy for pregnancy dating is 1:

1. First trimester ultrasound with CRL ≥10mm
2. Second trimester ultrasound before 24 weeks
3. LMP dating (only when cycles are certain and regular)
4. Sequential third trimester ultrasounds (last resort)

Clinical Considerations

• Early ultrasound dating (10-12 weeks) provides the most accurate prediction of delivery date 2
• Using ultrasound dating significantly reduces the number of pregnancies classified as post-term 3
• Combining multiple ultrasound measurements does not significantly improve dating accuracy over using CRL in first trimester or BPD in second trimester 3
• For pregnancies resulting from assisted reproductive technology, gestational age can be calculated based on embryo age and transfer date with high precision 6

Common Pitfalls

• Relying solely on LMP can lead to misclassification of gestational age in up to 26% of pregnancies 1
• Using dating formulas developed for second trimester may not perform well when applied to earlier gestations 7
• Dating accuracy decreases in cases of preterm birth, small-for-gestational-age fetuses, and multiple gestations 6
• After 24 weeks, ultrasound becomes progressively less reliable for dating purposes 2

By following these guidelines for pregnancy dating ultrasound, clinicians can optimize prenatal care planning, improve the accuracy of screening tests, and reduce unnecessary interventions for presumed post-term pregnancies.