What is the recommended approach for congenital anomaly scanning in a low-risk pregnant woman, particularly during the second trimester?

Congenital Anomaly Scanning Approach

For low-risk pregnant women, perform a comprehensive second-trimester anatomical ultrasound between 18-22 weeks' gestation as the primary screening modality, with consideration of an additional first-trimester scan at 11-14 weeks for early detection of lethal and severe anomalies. 1

Primary Screening Strategy

Second Trimester Anatomical Survey (18-22 weeks)

This is the gold standard screening examination that all low-risk women should receive. 1

• Timing: Perform between 18-22 weeks' gestation for optimal visualization of fetal anatomy 1
• Detection capability: Identifies approximately 50-55% of major malformations when performed as a single scan before 24 weeks 1
• Safety profile: Long-term follow-up studies demonstrate no detrimental effects on cognitive or physical development from ultrasound exposure 1

Key Anatomical Structures to Evaluate

The comprehensive survey must include systematic evaluation of:

• Central nervous system: Cranium, ventricles, cerebellum, posterior fossa 1
• Cardiovascular system: Four-chamber heart view, outflow tracts 1
• Thorax: Lungs, diaphragm 1
• Abdominal wall and organs: Stomach, kidneys, bladder, abdominal wall integrity 1
• Skeletal system: Spine, long bones, extremities 1
• Placenta and cord: Location, cord insertion sites, number of vessels 1

Enhanced Two-Stage Screening Approach

First Trimester Scan (11-14 weeks)

Consider adding an early anatomical survey at 11-14 weeks, particularly in centers with appropriately trained sonographers, as this detects 91.3% of lethal anomalies and 37.5% of all structural anomalies. 2, 3

Specific anomalies detectable in first trimester:

• Lethal anomalies: Acrania/anencephaly, severe neural tube defects 4, 2
• Severe structural defects: Omphalocele, megacystis, holoprosencephaly, cystic hygromata 4, 2
• Multiple severe congenital anomalies and severe skeletal dysplasias 2

Combined Screening Performance

Two-stage screening (first trimester + second trimester) achieves 83.8% sensitivity for detecting structural anomalies before 24 weeks, compared to only 50.5% sensitivity with single second-trimester screening alone. 3

• Specificity remains high: 99.9% for two-stage vs 99.8% for single-stage 3
• False-positive rate: Approximately 0.1% with two-stage screening 3
• Clinical advantage: Earlier detection allows for chorionic villus sampling if karyotyping needed and simpler termination procedures if chosen 5

Technical Considerations

Transabdominal vs Transvaginal Approach

• Primary method: Transabdominal ultrasound is the standard approach for second-trimester screening 6
• Transvaginal supplementation: Use transvaginal scanning only when transabdominal views are suboptimal due to maternal obesity, fetal position, or when structures lie close to the cervix 1
• Never use transvaginal as sole approach for second-trimester anatomical evaluation 1

Doppler Imaging

• Use selectively: Reserve Doppler for assessment of vascular anomalies and blood flow in fetal masses 1
• Safety precaution: Doppler produces highest acoustic energy output; use lowest energy level compatible with accurate diagnosis, especially in early gestation 1

Advanced Imaging Modalities

3-D/4-D ultrasound:

• Helpful for further evaluation of specific findings, particularly facial clefts 1

MRI without contrast:

• Reserve for cases where ultrasound is suboptimal or additional detail needed after abnormality detected on ultrasound 1
• Particularly valuable for fetal brain anomalies, lung volume measurement in diaphragmatic hernia, and pre-surgical planning 4
• Not indicated for routine screening 1

Integration with Aneuploidy Screening

First Trimester Combined Screening (11-14 weeks)

• Components: Nuchal translucency measurement + PAPP-A + hCG 1, 7
• Detection rate: 82-87% for Down syndrome with 5% false-positive rate 1
• Critical timing: Nuchal translucency must be measured between 11 weeks 4 days and 13 weeks 6 days 7

Cell-Free DNA (NIPT)

• Performance: 99% detection rate for trisomy 21 with 0.5% false-positive rate 1
• Limitation: Cannot replace ultrasound for structural anomaly detection 1
• Use in low-risk populations: Exercise caution due to higher false-positive rate from decreased pretest probability 1

Second Trimester Serum Screening

• Offer to women presenting in second trimester: AFP, hCG, unconjugated estriol, inhibin A (quad screen) 1
• Also offer to women who had first-trimester screening: MSAFP screening optimally at 16-18 weeks for neural tube defect detection 1

Management of Soft Markers

When isolated soft markers are identified on ultrasound:

After Negative Screening Results

• Echogenic intracardiac focus: No further evaluation needed; normal variant with no indication for echocardiography or follow-up 1
• Choroid plexus cysts: No further aneuploidy evaluation; normal variant requiring no follow-up 1
• Echogenic bowel, urinary tract dilation, shortened long bones: No further aneuploidy evaluation, but specific follow-up needed for non-aneuploidy concerns 1

Without Prior Screening

• Isolated echogenic intracardiac focus, echogenic bowel, urinary tract dilation, shortened humerus/femur: Counsel regarding trisomy 21 probability; offer cell-free DNA or quad screen 1
• Thickened nuchal fold or absent/hypoplastic nasal bone: Counsel regarding trisomy 21; discuss options including cell-free DNA, quad screen, or amniocentesis based on clinical circumstances 1
• Isolated choroid plexus cysts: Counsel regarding trisomy 18 probability; offer cell-free DNA or quad screen 1

Common Pitfalls and Limitations

Technical Limitations

• Maternal obesity: Significantly impairs visualization; consider transvaginal supplementation at 12-16 weeks 1
• Fetal position: May prevent adequate visualization of certain structures 1
• Oligohydramnios: Limits acoustic windows 1

Timing-Dependent Detection

Some anomalies cannot be detected until later in pregnancy due to ongoing organ development:

• Cardiac defects: Only 33.3% detected at early scan, 41.7% at 20-week scan, 25% after birth 2
• Gastrointestinal anomalies: Lowest detection rates across all screening approaches 3

Quality Assurance Requirements

• Sonographer training: First-trimester anatomical screening requires additional training beyond nuchal translucency certification 1, 2
• Adherence to standards: Follow ACR-SPR-SRU practice parameters for performing diagnostic ultrasound 1
• Quality control programs: Essential for maintaining detection rates and minimizing false positives 1

Follow-Up Imaging

Third Trimester Surveillance

Specific indications for third-trimester ultrasound:

• Isolated echogenic bowel: Reassessment and growth evaluation 1
• Shortened long bones: Reassessment and growth evaluation 1
• Single umbilical artery: Growth evaluation; consider weekly antenatal surveillance from 36 weeks 1
• Urinary tract dilation A1: Ultrasound at 32 weeks to determine need for postnatal follow-up 1

When Increased Nuchal Translucency Detected

NT ≥3.5 mm requires detailed anatomic ultrasound and/or echocardiogram, even with low-risk aneuploidy screen or normal karyotype. 1

• Associated with congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and genetic syndromes 1