When to Perform the Anatomy Scan
The anatomy ultrasound should be performed between 18 and 22 weeks' gestation, with multi-specialty consensus (ACR-ACOG-AIUM-SMFM-SRU) specifically recommending that at least one ultrasound be offered to all pregnant women between 18 and 20 weeks. 1, 2
Standard Timing for Low-Risk Pregnancies
The optimal window is 18-20 weeks' gestation for the standard fetal anatomical examination, as this timing provides the best balance between fetal size for visualization and adequate time for clinical decision-making if abnormalities are detected. 1, 3
The anatomy scan remains acceptable through 22 weeks' gestation, though screening performance may begin to decline after the optimal 18-20 week window. 3
Visualization rates improve significantly with advancing gestational age: complete anatomic surveys are achieved in 67% of cases at 16-18 weeks, 82% at 18-20 weeks, and 96% at 20-22 weeks. 4
Do not delay the scan beyond 22 weeks, as this limits clinical options if abnormalities requiring specialized delivery planning are detected. 5, 3
Modified Timing for Special Populations
Obese Patients (BMI ≥35 kg/m²)
Schedule the anatomic survey at 20-22 weeks (approximately 2 weeks later than normal weight women) due to suboptimal visualization related to increased body mass index. 1, 3
If the initial scan is incomplete, schedule a repeat follow-up ultrasound in 2-4 weeks. 1
Consider early transvaginal imaging combined with the routine transabdominal study at 18-22 weeks to achieve completion rates comparable to non-obese populations. 1
Multiple Gestations
For both dichorionic and monochorionic twins, perform the fetal anatomy scan at 18-22 weeks' gestation, consistent with singleton pregnancy recommendations. 1, 6
Monochorionic twins require additional surveillance starting at 16 weeks' gestation, occurring at least every 2 weeks, to monitor for twin-specific complications (TTTS, TRAP, TAPS, selective fetal growth restriction). 1
Dichorionic twins should have serial ultrasound assessment every 3-4 weeks starting from the anatomy scan. 6
High-Risk Pregnancies
High-risk patients (advanced maternal age, in vitro fertilization, drug dependence, infection, maternal medical conditions, abnormal screening tests, family history of genetic disease, teen pregnancies) should have detailed fetal anatomic examinations at the standard 18-22 week window. 1
Teen pregnancies warrant particular attention, with 9.2% showing anomalies or aneuploidy. 1
Key Anatomical Structures to Evaluate
The standard examination at 18-22 weeks should include comprehensive evaluation of:
Cranial anatomy: lips, face, midline, ventricles, choroid plexus, cerebellum (98% visualization within 30 minutes). 4
Spine: complete visualization (91% within 30 minutes). 4
Cardiac screening: four-chamber view, aortic and pulmonary outflow views (91% within 30 minutes). 4
Abdominal anatomy: stomach, kidneys, bladder, ventral wall, three-vessel cord (99% within 30 minutes). 4
Cervical assessment: cervical length, placenta location, vasa previa screening. 1, 5
Common Pitfalls to Avoid
Do not assume that earlier scanning (12-17 weeks) can replace the standard 18-22 week anatomy scan, even though some structures may be visible earlier. While early anatomy ultrasound shows 83.3% sensitivity for major anomalies, cardiac anatomy remains particularly difficult to assess before 18 weeks. 7, 8
Do not schedule routine anatomy scans before 18 weeks in average-risk patients, as completion rates are significantly lower (only 23.1% at 11-11+6 weeks versus 63.8% at 13+3-13 weeks). 7
Do not rely solely on last menstrual period dating—use composite ultrasound measurements from the anatomy scan to establish accurate gestational age, as ultrasound dating is more accurate and improves screening sensitivity and specificity. 3
Do not perform the anatomy scan as the sole ultrasound approach using transvaginal technique—transabdominal imaging should be the primary method, with transvaginal reserved only for incomplete or suboptimal transabdominal evaluation. 1, 2