What is the recommended guideline for congenital anomaly scanning in a pregnant individual?

Congenital Anomaly Scanning Guidelines

Primary Screening Recommendation

For all pregnant individuals, perform a transabdominal ultrasound anatomy scan at 18-22 weeks' gestation as the primary screening for congenital anomalies. 1

This timing represents the optimal window when fetal structures are adequately developed for visualization while still allowing time for comprehensive counseling and decision-making if anomalies are detected.

---

Timing Algorithm by Risk Status

Low-Risk Pregnancies

• Standard anatomy scan: 18-20 weeks' gestation 1, 2
• This single comprehensive scan is usually appropriate for initial screening 1
• The American College of Radiology specifically recommends 18-20 weeks as the primary screening window 2

High-Risk Pregnancies

• Detailed ultrasound scan: 18-22 weeks' gestation 1
• Consider earlier transvaginal ultrasound at 12-16 weeks as an adjunct (not replacement) for high-risk patients 2
• High-risk factors include: maternal genetic disease, current medical conditions, chemical exposures, known fetal anomaly, or cardiac abnormality 1

Obese Patients (Critical Caveat)

• Delay anatomy scan to 20-22 weeks due to suboptimal visualization through increased soft tissue 2
• Schedule repeat follow-up in 2-4 weeks if initial scan is incomplete 2

---

Structured Examination Components

Essential Views and Measurements

The anatomy scan must systematically evaluate:

• Central nervous system: brain structures, spine for neural tube defects 1
• Facial structures: cleft lip and palate (lip defects more readily visible than isolated palate) 1, 2
• Cardiac structures: four-chamber view, outflow tracts 1
• Thoracic structures: lungs, diaphragm for hernias, congenital pulmonary airway malformations 1
• Abdominal structures: stomach, kidneys, bladder, abdominal wall (gastroschisis, omphalocele) 1
• Skeletal structures: long bones, digits for skeletal dysplasias 1
• Placenta and cord: insertion sites, vasa previa assessment 1
• Amniotic fluid volume: polyhydramnios or oligohydramnios 1
• Estimated fetal weight: calculate and document 1

---

Management Algorithm for Abnormal Findings

When Soft Markers Are Detected

Soft markers (echogenic intracardiac focus, choroid plexus cyst, renal pyelectasis, short humerus/femur, nuchal thickening, echogenic bowel, short/absent nasal bone):

1. Perform detailed ultrasound scan at the same visit or schedule for near future 1
2. For aneuploidy-only markers (echogenic intracardiac focus, choroid plexus cyst): detailed scan is optional if finding appears isolated 1
3. For other soft markers: detailed scan is usually indicated to ensure finding is truly isolated 1
4. Schedule follow-up ultrasound as clinically appropriate 1

When Major Anomalies Are Detected

Major anomalies (hydrops fetalis, CNS malformations, facial clefts, diaphragmatic hernias, gastroschisis, skeletal dysplasias, cardiac defects):

1. Immediately perform or schedule detailed ultrasound scan according to AIUM Consensus Report 1
2. Order fetal echocardiography if cardiac abnormality suspected or other risk factors present 1
3. Consider fetal MRI without contrast when:
   • Ultrasound information is incomplete 1
   • CNS anomalies require better characterization 1
   • Planning prenatal/postnatal intervention 1
   • Airway management needed for neck masses 1
   • Isolated ventriculomegaly, corpus callosum agenesis, absent cavum septi pellucidi, or cerebellar/vermian anomalies detected 1
4. Optimal MRI timing: at or after 22 weeks' gestation (18-22 weeks may be valuable in certain settings) 1
5. Schedule follow-up ultrasound for growth monitoring, delivery planning, and postnatal management 1

---

Specialized Cardiac Screening

Cardiac-Specific Timing

• Optimal window for cardiac screening: 18-22 weeks' gestation when heart and outflow tracts are best visualized 1
• Visualization becomes more difficult after 30 weeks due to fetal crowding 1

Indications for Fetal Echocardiography

Perform specialized fetal echocardiography when:

• Maternal genetic disease or cardiac history 1
• Maternal medical conditions (diabetes, autoimmune disease) 1
• Maternal chemical/medication exposures 1
• Abnormal cardiac screening on anatomy scan 1
• Other fetal anomaly detected (increased cardiac risk) 1

---

Multiple Gestations (Special Considerations)

Monochorionic Twins

• Begin surveillance at 16 weeks' gestation with scans every 2 weeks minimum 1
• Perform anatomy scan at 18-22 weeks as with singletons 1
• Assess for twin-specific complications: TTTS, TAPS, TRAP sequence, selective fetal growth restriction 1
• Document placental cord insertions: velamentous insertion present in up to 22% of monochorionic twins and increases adverse outcomes 1
• Perform fetal echocardiography: 2% risk in uncomplicated monochorionic twins, 5% with TTTS, 8-fold increased risk in monoamniotic twins 1
• Evaluate for vasa previa: higher frequency with velamentous cord insertion 1

Dichorionic Twins

• Standard anatomy scan at 18-22 weeks 1
• Document estimated fetal weight for each twin at every scan 1
• Assess for vasa previa and velamentous cord insertion (more common in multiples) 1

---

First Trimester Scanning (Supplementary)

Early Detection Capabilities

• 12-13 week scan can detect approximately 63.5% of all anomalies (45% of structural, 100% of chromosomal with increased NT or structural findings) 3
• All particularly severe anomalies detectable early: neural tube defects, omphalocele, megacystis, multiple severe congenital anomalies 3
• First trimester cardiac detection: 85% sensitivity, 99% specificity for major congenital heart disease 1

When to Perform Early Scanning

• High-risk patients with specific indications 2
• Suspected specific anomaly requiring early evaluation 4
• As adjunct (not replacement) to standard 18-22 week scan 2

---

Advanced Imaging: Fetal MRI

Indications for MRI

Fetal MRI without IV contrast is indicated when:

• Ultrasound findings are incomplete or inconclusive 1
• CNS anomalies require detailed characterization 1
• Cranial, facial, thoracic, abdominal, retroperitoneal, or pelvic anomalies need further evaluation 1
• Monochorionic gestation complications present 1
• Parental reassurance needed for apparently isolated conditions 1

MRI Timing and Safety

• Ideal timing: at or after 22 weeks' gestation 1
• 18-22 weeks may be valuable in certain clinical settings 1
• Avoid gadolinium contrast: crosses placenta, long-term fetal risks unknown 1
• MRI without contrast is safe, especially after first trimester 1

---

Critical Pitfalls to Avoid

Common Errors

1. Scanning obese patients at 18 weeks: Delay to 20-22 weeks for adequate visualization 2
2. Assuming isolated soft markers are benign: Always perform detailed scan for markers beyond echogenic intracardiac focus and choroid plexus cyst 1
3. Missing cardiac anomalies: Only 33-42% of heart defects detected at early or routine scans; maintain high index of suspicion 3
4. Overlooking vasa previa in multiples: Specifically evaluate when velamentous cord insertion present 1
5. Performing MRI too early: Wait until 22 weeks for optimal fetal brain imaging 1
6. Using gadolinium contrast: Should be avoided in pregnancy 1

Incomplete Examination Management

• If study is incomplete: bring patient back for focused reassessment 1
• Document reasons for incomplete examination: maternal body habitus, fetal position, oligohydramnios 1
• Schedule appropriate follow-up interval: typically 2-4 weeks 2

---

Follow-Up Surveillance

Ongoing Monitoring After Anomaly Detection

• Serial ultrasounds for growth assessment if pregnancy continues 1
• Delivery planning coordination with appropriate specialists 1
• Postnatal management preparation with pediatric subspecialists 1
• Repeat imaging as clinically indicated for evolving or progressive anomalies 1

Normal Anatomy Scan Follow-Up

• No routine follow-up imaging needed after normal low-risk anatomy scan 1
• Third trimester growth scan may be indicated for specific clinical indications (not routine anomaly screening) 1