When should women get a dating ultrasound and first trimester bloodwork done by which week of gestation?

Timing of Dating Ultrasound and First Trimester Bloodwork in Pregnancy

Women should undergo dating ultrasound between 11-13+6 weeks gestation, with optimal timing at 11 completed weeks, and first trimester bloodwork should be completed during the same timeframe to maximize screening accuracy and clinical utility. 1

Dating Ultrasound Timing

First Trimester Dating (11-13+6 weeks)

• First trimester ultrasound provides the most accurate pregnancy dating with a 95% confidence range of ±5 days 2
• Crown-rump length (CRL) measurement between 45-84mm (corresponding to 11-13+6 weeks) is optimal for:
  • Accurate dating
  • Nuchal translucency (NT) measurement for aneuploidy screening
  • Determination of chorionicity in multiple gestations 3, 1
• Earlier dating (before 11 weeks) may be performed but is less optimal for NT measurement 1

Benefits of First Trimester Dating

• Confirms pregnancy viability
• Establishes accurate gestational age for subsequent screening and management decisions
• Identifies multiple gestations early
• Allows for limited structural survey to detect major anomalies 4
• Provides baseline for growth assessment in later pregnancy 5

First Trimester Bloodwork Timing

Optimal Timing (11-13+6 weeks)

• First trimester aneuploidy screening should include:
  • Maternal serum pregnancy-associated plasma protein A (PAPP-A)
  • Free beta-hCG or total/intact hCG 1
• These markers should be measured at the same time as NT ultrasound (11-13+6 weeks) for integrated risk assessment
• Clinical sensitivity decreases after 13 completed weeks' gestation 1

Important Considerations

• PAPP-A measurements are more discriminatory earlier in the first trimester 3
• Ultrasound dating is strongly preferred over last menstrual period (LMP) dating for accurate interpretation of serum markers 3
• Using LMP dating may result in inappropriately high screen-positive rates and is not recommended 3

Screening Options and Follow-Up

Integrated Screening

• Full integrated test combines:
  • First trimester: NT measurement and PAPP-A
  • Second trimester: AFP, hCG, uE3, and inhibin-A 3
• Provides highest detection rate (92% at 5% false-positive rate) 3

Sequential Screening

• First trimester results categorized as:
  • High risk: Offered immediate diagnostic testing
  • Intermediate risk: Proceed to second trimester testing
  • Low risk: Reassurance provided 3

Second Trimester Follow-Up

• Women should be offered MSAFP screening for neural tube defects at 16-18 weeks, even if they had first trimester screening 3
• Multiple marker screening (quad screen) should be completed at 16-18 weeks unless amniocentesis is indicated or first trimester screening was performed 3

Common Pitfalls to Avoid

1. Dating errors: Relying on LMP rather than ultrasound dating can lead to inaccurate risk assessment
2. Missed timing window: Performing NT ultrasound after 13+6 weeks reduces screening accuracy
3. Incomplete follow-up: Failing to complete second trimester screening for neural tube defects after first trimester screening
4. Inadequate patient education: Not providing information about available screening options and their limitations

By adhering to the 11-13+6 week timeframe for both dating ultrasound and first trimester bloodwork, clinicians can optimize screening accuracy and provide women with the most reliable information for making informed decisions about their pregnancy care.