What is the purpose and protocol for a mid trimester anatomy scan (ultrasound scan between 16 and 20 weeks of gestation) for a pregnant woman with no known medical history or complications?

Mid-Trimester Anatomy Scan

All pregnant women should be offered at least one comprehensive transabdominal ultrasound between 18 and 20 weeks of gestation to screen for fetal structural anomalies. 1

Purpose of the Mid-Trimester Anatomy Scan

The mid-trimester anatomy scan serves as the primary screening tool for detecting fetal structural abnormalities in low-risk pregnancies. 1 This timing represents a multi-specialty consensus recommendation from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, with standardized components agreed upon by ACR-ACOG-AIUM-SMFM-SRU. 1

The scan aims to:

• Detect major structural anomalies across all organ systems before 24 weeks when clinical management options remain available 1
• Provide accurate gestational age dating using composite fetal measurements (accurate within 10 days in the second trimester) 2
• Assess placental location, amniotic fluid volume, and cervical length 1
• Screen for neural tube defects when combined with maternal serum alpha-fetoprotein (MSAFP) testing 1

Standard Protocol for Low-Risk Pregnancies

Optimal Timing

Schedule the anatomy scan between 18-20 weeks' gestation for standard visualization of fetal structures. 1, 2 Research demonstrates that complete anatomic surveys improve significantly with gestational age, rising from 67% completion at 16-18 weeks to 96% at 20-22 weeks. 3 The scan remains acceptable through 20 weeks 6 days, though performance may decline after this window. 2

Technical Approach

Transabdominal ultrasound is the primary modality for the mid-trimester anatomy scan. 1, 4 Transvaginal ultrasound should only be added if transabdominal evaluation is suboptimal or incomplete—never as the sole approach for second-trimester anatomy evaluation. 1

Required Anatomic Survey Components

The examination must systematically evaluate: 1

• Cranial structures: ventricles, midline, cerebellum, choroid plexus, face, lips
• Spine: complete visualization in sagittal and axial planes
• Cardiac screening: four-chamber view, aortic and pulmonary outflow tracts
• Abdominal anatomy: stomach, kidneys, bladder, ventral wall, three-vessel cord
• Extremities: presence and length of long bones
• Placenta and amniotic fluid: location, appearance, volume assessment

Time Requirements

A comprehensive anatomic survey requires adequate scanning time. 3 Studies show that only 8% of complete surveys are achieved within 10 minutes, increasing to 31% by 15 minutes, 53% by 20 minutes, and 81% by 30 minutes. 3 Providers should allocate at least 20-30 minutes for thorough evaluation to maximize detection rates.

Special Considerations

Obese Patients (BMI ≥35 kg/m²)

Delay the anatomy scan to 20-22 weeks in obese women (approximately 2 weeks later than standard timing) due to suboptimal visualization from soft tissue attenuation. 1 If the initial scan is incomplete, schedule a repeat follow-up ultrasound in 2-4 weeks. 1 Emerging evidence supports adding early transvaginal imaging at 12-16 weeks combined with the standard transabdominal study at 18-22 weeks to achieve completion rates comparable to non-obese populations. 1

Incomplete Examinations

When cardiac or renal structures cannot be adequately visualized (occurring in up to 42% and 27% of cases respectively at earlier gestational ages), schedule a repeat scan 2-4 weeks later rather than proceeding with an incomplete evaluation. 5, 3

Dating Uncertainty

Use composite ultrasound measurements from the anatomy scan to establish accurate gestational age when last menstrual period (LMP) dating is uncertain. 2 Ultrasound dating improves both sensitivity and specificity of screening compared to LMP dating alone. 2 In the second trimester, biparietal diameter (BPD) measurement is specifically recommended as it can rule out anencephaly and improve detection of open spina bifida. 2

Detection Rates and Limitations

Two-stage screening (first-trimester plus second-trimester scans) detects 83.8% of structural anomalies overall with 99.9% specificity, compared to 50.5% sensitivity for single second-trimester screening alone. 6 However, for low-risk women presenting after the first trimester, the single mid-trimester scan remains the standard approach. 1

The scan demonstrates highest sensitivity for:

• Thoracic and abdominal wall anomalies 6
• Lethal anomalies (91.3% detection in first trimester, higher in second trimester) 6

Lowest sensitivity occurs for:

• Gastrointestinal anomalies 6
• Cardiac defects (particularly complex lesions requiring specialized fetal echocardiography) 1

Common Pitfalls to Avoid

Do not delay the scan beyond 22 weeks thinking additional time will improve visualization—this limits clinical options if abnormalities are detected. 2 The optimal window closes at approximately 22 weeks. 1, 2

Do not rely solely on LMP dating for pregnancy management—composite ultrasound measurements provide superior accuracy. 2

Do not use specialized imaging modalities (MRI, fetal echocardiography, 3D/4D ultrasound) for routine screening in low-risk pregnancies—these are reserved for evaluation of suspected or confirmed anomalies. 1

Do not skip MSAFP screening in women who undergo first-trimester screening or chorionic villus sampling—neural tube defect screening remains indicated between 16-20 weeks. 1

Follow-Up Recommendations

No routine third-trimester anatomy scan is indicated for low-risk pregnancies with normal mid-trimester findings. 1 Large trials with 34,980 patients showed no evidence of improved outcomes with routine third-trimester screening. 1

However, selective third-trimester ultrasound may be appropriate for:

• Late presentation for prenatal care 1
• Specific clinical indications (growth concerns, decreased fetal movement, etc.)
• Follow-up of borderline or evolving findings from the mid-trimester scan 1