What evaluations and interventions should be performed at the 20-week obstetric (OB) visit?

20-Week Obstetric Visit: Comprehensive Evaluation Protocol

At the 20-week obstetric visit, a comprehensive ultrasound examination should be performed to evaluate fetal anatomy, assess maternal health, and screen for potential pregnancy complications. 1, 2

Ultrasound Examination

Standard Anatomic Scan

• Complete fetal anatomic survey between 18-22 weeks (optimal timing at 20 weeks) 1, 2
• Evaluation should include:
  • Fetal biometry (head circumference, biparietal diameter, abdominal circumference, femur length)
  • Four-chamber view of heart and outflow tracts
  • Detailed brain evaluation
  • Spine assessment in longitudinal and transverse planes
  • Abdominal wall evaluation for defects
  • Assessment of extremities including hands and feet
  • Amniotic fluid volume assessment
  • Placental location and appearance 1, 2

Special Considerations

• For obese patients, consider scheduling the scan at 20-22 weeks (about 2 weeks later than normal weight women) 1
  • If incomplete, schedule follow-up in 2-4 weeks
  • Completion rates are significantly higher when performed between 18-20 weeks compared to 15-18 weeks for most BMI categories 3
• For multiple gestations:
  • Confirm chorionicity if not previously determined
  • Evaluate for discordant growth
  • Assess for structural anomalies (higher risk in multiple gestations) 1
  • Consider fetal echocardiography (especially for monochorionic twins or twins conceived through assisted reproductive technologies) 1

Maternal Assessment

Pre-eclampsia Screening

• Blood pressure measurement (using proper technique)
• Urine protein assessment
• Assessment for symptoms of pre-eclampsia:
  • Headache or visual disturbances
  • Epigastric pain or vomiting
  • Reduced fetal movements 1

Cervical Length Assessment

• Consider transvaginal cervical length measurement, especially for high-risk patients
• For twin pregnancies, cervical length <1.5 cm between 15-24 weeks is predictive of preterm labor 1

Additional Evaluations

High-Risk Pregnancy Assessment

• For high-risk pregnancies (advanced maternal age, IVF pregnancy, drug dependence, infection, abnormal screening tests, family history of genetic disease, multi-gestational pregnancies):
  • Consider more detailed anatomic examination
  • Consider genetic counseling and testing if indicated 1, 2

Fetal Growth Assessment

• Evaluate for fetal growth restriction (FGR) using population-based references
• If FGR is detected (estimated fetal weight or abdominal circumference below 10th percentile):
  • Consider additional management including detailed ultrasound and genetic testing 2

Patient Education and Follow-up Planning

• Discuss ultrasound findings with patient
• Educate about signs and symptoms of pre-eclampsia and when to seek medical attention 1
• Schedule appropriate follow-up visits based on risk factors:
  • Women with no risk factors: follow standard schedule
  • Women with one pre-eclampsia risk factor: visits every 2-3 weeks until delivery 1

Common Pitfalls to Avoid

1. Inadequate ultrasound assessment: Ensure complete visualization of all required anatomic structures; document any limitations.
2. Improper blood pressure measurement: Errors in BP measurement have been implicated in maternal deaths.
3. Missing multiple gestation diagnosis: Careful evaluation is needed to confirm chorionicity and amnionicity.
4. Overlooking maternal obesity impact: Maternal obesity can limit visualization and reduce detection of fetal anomalies; schedule appropriate follow-up if views are suboptimal.
5. Failure to recognize high-risk factors: Identify patients needing more intensive monitoring or specialist referral.

Remember that while the 20-week ultrasound is a critical screening tool, it cannot detect all anomalies even with optimal imaging 2. Proper documentation of findings and appropriate follow-up of any concerns are essential for optimal maternal and fetal outcomes.