What workup should be ordered at 18 weeks of pregnancy?

Recommended Workup at 18 Weeks of Pregnancy

At 18 weeks of pregnancy, a comprehensive transabdominal ultrasound anatomy scan should be performed as the standard of care for all pregnant women to screen for fetal anomalies and assess maternal health. 1, 2

Standard Ultrasound Anatomy Scan Components

The 18-20 week ultrasound should include:

• Fetal biometry measurements:

  • Head circumference
  • Biparietal diameter
  • Abdominal circumference
  • Femur length
  • Estimated fetal weight

• Detailed anatomical assessment:

  • Brain: ventricles, cerebellum, cisterna magna
  • Face: profile, orbits, lips
  • Spine: longitudinal and transverse views
  • Heart: four-chamber view and outflow tracts
  • Chest: lungs, diaphragm
  • Abdomen: stomach, kidneys, bladder, umbilical cord insertion
  • Extremities: arms, legs, hands, feet
  • Sex determination (if desired)

• Maternal assessment:

  • Placental location and appearance
  • Amniotic fluid volume
  • Cervical length (transabdominal, with transvaginal if indicated)

Additional Assessments

• Blood pressure measurement using proper technique
• Urine protein assessment to screen for pre-eclampsia 2
• Patient education about signs and symptoms of pre-eclampsia and when to seek medical attention 2

Special Considerations

For Obese Patients

• Consider scheduling the anatomy scan at 20-22 weeks (about 2 weeks later than women of normal weight)
• If incomplete visualization, schedule follow-up in 2-4 weeks 1, 2

For Multiple Gestations

• Confirm chorionicity if not previously determined
• Evaluate for discordant growth
• Consider fetal echocardiography, especially for monochorionic twins
• Assess umbilical cord insertions (especially important in monochorionic twins)
• Evaluate for twin-twin transfusion syndrome in monochorionic pregnancies 1
• Consider cervical length measurement (cervical length <1.5 cm between 15-24 weeks is predictive of preterm labor in twin pregnancies) 1, 2

For High-Risk Pregnancies

• More detailed anatomic examination for patients with:

  • Advanced maternal age
  • Abnormal screening tests
  • Family history of genetic disease
  • History of prior affected pregnancy
  • Use of in vitro fertilization
  • Drug dependence
  • Maternal medical conditions 1

• Consider amniocentesis for:

  • Advanced maternal age
  • Abnormal screening results
  • Abnormal ultrasound findings 2, 3

Important Caveats

• Ultrasound should be used only when medically indicated using the lowest acoustic energy level compatible with accurate diagnosis (ALARA principle) 1
• Not all anomalies can be detected, even with optimal imaging 2
• Some anomalies may only be detectable later in pregnancy 1
• While early anatomy scans (12-17 weeks) can detect many anomalies, the standard 18-22 week scan remains essential as subtle anomalies, particularly of the heart, remain difficult to diagnose early 4, 5, 6

By following this comprehensive approach at 18 weeks, clinicians can effectively screen for fetal anomalies, assess maternal health, and identify pregnancies that may require additional monitoring or intervention to optimize outcomes.