At what age of gestation is congenital (fetal anomaly) scanning typically performed?

Timing of Congenital Anomaly Scanning

The standard congenital anomaly scan (fetal anatomy scan) is performed at 18-22 weeks' gestation, with the optimal window being 18-20 weeks for low-risk pregnancies. 1, 2, 3

Standard Screening Protocol

For routine prenatal care, schedule the comprehensive fetal anatomy scan between 18-22 weeks' gestation using transabdominal ultrasound. 1, 2, 3 This timing represents multi-specialty consensus from the American College of Radiology, American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Society for Maternal-Fetal Medicine, and Society of Radiologists in Ultrasound. 3

Low-Risk Pregnancies

• Target 18-20 weeks' gestation as the primary screening window 2, 3
• This timing optimizes visualization of fetal structures while allowing sufficient time for decision-making if anomalies are detected 2

High-Risk Pregnancies

• Perform detailed ultrasound at 18-22 weeks' gestation 2
• Consider earlier transvaginal ultrasound at 12-16 weeks as an adjunct screening tool 2
• Earlier scanning does not replace the standard 18-22 week anatomy scan 2

First Trimester Screening (11-14 Weeks)

While not the primary congenital anomaly scan, first trimester ultrasound at 11-14 weeks can detect approximately 45% of structural anomalies and all chromosomal anomalies presenting with structural findings or increased nuchal translucency. 4

• Nuchal translucency measurement is optimally performed at 11-14 weeks' gestation 1
• Particularly severe anomalies detectable at this stage include neural tube defects, omphalocele, megacystis, and multiple severe skeletal anomalies 4
• First trimester scanning does not replace the 18-22 week anatomy scan, even when early findings are normal 4

Cardiac-Specific Timing

The optimal window for cardiac evaluation is 18-22 weeks' gestation when cardiac structures and outflow tracts are best visualized. 1, 2

High-Risk Cardiac Screening

• For fetuses with nuchal translucency ≥3 mm or 99th percentile, perform fetal echocardiography at 18-22 weeks 1
• Early echocardiography at 11-14 weeks can identify 50-65% of major heart anomalies but requires follow-up at 18-22 weeks 1
• A complete cardiac evaluation is possible at 13-14 weeks in over 90% of cases, but repeat evaluation in the second trimester remains mandatory 1

Critical Timing Considerations

Do Not Delay Beyond 22 Weeks

• Scanning after 22 weeks limits clinical options if abnormalities are detected 3
• The anatomy scan remains acceptable through 20 weeks 6 days, though performance may decline after 20 weeks 3

Special Population Adjustments

• For obese patients (BMI ≥35 kg/m²), schedule at 20-22 weeks rather than 18 weeks due to suboptimal visualization 2, 3
• Scanning obese patients at 18 weeks frequently results in inadequate visualization requiring repeat examination 2

Multiple Gestations

Monochorionic Twins

• Begin surveillance at 16 weeks with scans every 2 weeks minimum 2
• Perform standard anatomy scan at 18-22 weeks as with singleton pregnancies 2

Dichorionic Twins

• Standard anatomy scan at 18-22 weeks is recommended 2

Common Pitfalls to Avoid

• Do not assume a normal first trimester scan eliminates the need for 18-22 week anatomy scanning 4 - many anomalies, particularly cardiac defects, are not detectable until the second trimester 5
• Do not scan obese patients at 18 weeks expecting adequate visualization 2, 3 - delay to 20-22 weeks to avoid incomplete examinations
• Do not rely solely on last menstrual period dating 3 - use composite ultrasound measurements for accurate gestational age determination, which improves screening sensitivity and specificity 3
• Do not delay scanning beyond 22 weeks waiting for a "perfect" appointment time 3 - this unnecessarily limits options if anomalies are found

Incomplete Examinations

If the anatomy survey is incomplete due to fetal position or maternal body habitus, schedule repeat ultrasound in 2-4 weeks. 3