When is a dating ultrasound (US) performed?

Dating Ultrasound Timing and Recommendations

A dating ultrasound should be performed in the first trimester, ideally between 7-14 weeks of gestation, to establish accurate pregnancy dating, with earlier scans (before 24 weeks) providing superior accuracy compared to later ultrasound assessments. 1, 2

Optimal Timing for Dating Ultrasound

• First trimester (7-14 weeks): This is the recommended timeframe for dating ultrasounds according to multiple guidelines 1, 2

  • Crown-rump length measurement during this period provides accuracy within ±7 days
  • First trimester ultrasound dating is superior to Last Menstrual Period (LMP) dating

• Early first trimester (5-7 weeks): While not the standard recommendation, research suggests that ultrasounds performed as early as 5-6 weeks may still provide reasonably accurate dating 3

• Before 24 weeks: Dating ultrasounds performed before 24 weeks are generally reliable, with earlier scans providing greater accuracy 4

Hierarchy for Pregnancy Dating

The American College of Obstetricians and Gynecologists recommends the following hierarchy for pregnancy dating 1, 2:

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

Special Circumstances Requiring Dating Ultrasound

1. Multiple gestations:

   • Dating is critical in twin pregnancies to establish chorionicity and amnionicity 5
   • Multiple gestations require more frequent ultrasound examinations than singleton pregnancies 5

2. Uncertain LMP:

   • When the last menstrual period is uncertain or irregular 1
   • First-trimester ultrasound with crown-rump length measurement provides more accurate dating than LMP alone 6

3. Planned interventions:

   • When planning elective induction of labor at 39 weeks 5
   • Accurate dating is imperative to avoid iatrogenic early term or preterm delivery 5

4. Periviable births:

   • Accurate gestational age is critical when managing pregnancies at risk of periviable birth 5
   • Ultrasound dating performed at less than 24 weeks of gestation estimates gestational age within 5 to 14 days 5

Clinical Implications of Dating Accuracy

• Dating ultrasound improves prediction of delivery date compared to LMP alone, with up to 92% of deliveries occurring within 37-42 weeks when gestational age is derived from ultrasound data, compared with 87% based on LMP 4

• The earlier the ultrasound assessment, the more accurate the prediction of delivery date, with optimal timing between 10-12 weeks 4

• After 24 weeks of gestation, a reliable LMP may provide better estimates of gestational age than ultrasound 4

Common Pitfalls to Avoid

• Changing established dates: Once gestational age is determined from first accurate ultrasound, subsequent changes should be reserved for rare circumstances 2

• Relying solely on LMP: This method assumes accurate recollection and regular 28-day cycles with ovulation on day 14, which is often not the case 6

• Late dating: Ultrasounds performed after 24 weeks have decreased accuracy for dating purposes 4

• Failure to document: The gestational age and estimated due date should be determined, discussed with the patient, and clearly documented in the medical record 2