Fetal Anomaly Scan Timing and Structures Evaluated
All pregnant women should be offered at least one transabdominal ultrasound anatomy scan between 18-20 weeks of gestation to screen for fetal structural anomalies. 1
Standard Timing for Low-Risk Pregnancies
The multi-specialty consensus from the Eunice Kennedy Shriver National Institute of Child Health and Human Development recommends offering routine ultrasound screening between 18-20 weeks' gestation for all pregnant women 1. This timing represents the optimal window for visualizing fetal anatomy while allowing sufficient time for decision-making if anomalies are detected.
Structures Evaluated During Standard Anatomy Scan
The standard fetal examination at 18-20 weeks includes comprehensive evaluation of:
- Central nervous system: Brain structures, ventricles, posterior fossa, spine
- Facial structures: Profile, orbits, lips/palate assessment
- Thoracic structures: Four-chamber heart view, outflow tracts, lungs, diaphragm
- Abdominal structures: Stomach, kidneys, bladder, abdominal wall, cord insertion
- Skeletal system: Long bones (femur, humerus), hands, feet, spine
- Placenta and amniotic fluid: Location, volume assessment
Modified Timing for Special Populations
Obese Patients
Perform the anatomic survey at 20-22 weeks (approximately 2 weeks later than standard timing) in obese women due to decreased visualization with increasing body mass index 1. If the examination is incomplete, schedule a repeat follow-up ultrasound in 2-4 weeks 1.
Emerging evidence supports combining early transvaginal imaging with the routine transabdominal study at 18-22 weeks to achieve completion rates comparable to non-obese populations 1.
High-Risk Pregnancies
High-risk patients require a detailed transabdominal scan rather than a routine anatomy scan 1. High-risk categories include:
- Advanced maternal age
- In vitro fertilization
- Drug dependence or specific medication exposures
- Maternal infections or medical conditions
- Abnormal screening tests (quad screen, NIPT, prior ultrasound findings)
- Family history of genetic disease
- Multi-gestational pregnancies
- Teen pregnancies (9.2% anomaly rate) 1
Detection Capabilities by Gestational Age
First Trimester (11-14 weeks)
While not standard for routine screening, first-trimester detailed scans can detect:
- 91.3% of lethal anomalies
- 37.5% of all structural anomalies overall
This approach is particularly valuable for detecting severe anomalies like neural tube defects, omphalocele, and megacystis early in pregnancy 3.
Second Trimester (18-24 weeks)
Two-stage screening (first + second trimester combined) detects 83.8% of structural anomalies before 24 weeks, compared to 50.5% with single-stage second-trimester screening alone 2. However, direct comparison studies are lacking, creating uncertainty about the magnitude of benefit.
Third Trimester
Third-trimester ultrasound may be performed selectively or for late arrivals, though evidence shows no improvement in antenatal, obstetric, or neonatal outcomes compared to controls 1. One study found 15% of anomalies were detected only in the third trimester, suggesting utility for perinatal management planning 1.
Follow-Up Imaging When Anomalies Are Suspected
For Soft Markers
When minor sonographic findings are identified (echogenic intracardiac focus, choroid plexus cysts, renal pyelectasis, short long bones, nuchal thickening, echogenic bowel), perform a detailed ultrasound examination to evaluate for additional markers and major anomalies 1.
For Major Anomalies
When major structural anomalies are detected, the following complementary studies are appropriate:
- Detailed ultrasound scan (if not already performed)
- Fetal MRI without contrast at ≥22 weeks (ideally), particularly for CNS anomalies, thoracic/abdominal pathology, and airway management planning 1
- Fetal echocardiography when cardiac abnormalities are suspected or risk factors present 1
- Follow-up ultrasound for growth monitoring and delivery planning 1
Critical Caveats
MRI with IV contrast is not recommended for fetal evaluation - there are no documented fetal indications for contrast use 1.
Doppler imaging requires caution, especially early in gestation, due to higher acoustic energy levels. Use only when clinically indicated with the lowest energy compatible with accurate diagnosis 1.
The accuracy of soft markers for adjusting trisomy risk may be less reliable than initially reported, particularly in the era of first-trimester screening and cell-free fetal DNA testing 1.