Level 1 and Level 2 Ultrasound Scans in Pregnancy
Level 1 ultrasound is a basic scan that estimates gestational age through fetal measurements, while Level 2 ultrasound is a detailed anatomical examination of the fetus to detect structural abnormalities. 1
Level 1 Ultrasound (Basic Scan)
Level 1 ultrasound focuses on fundamental pregnancy assessment 1:
- Primary purpose: Estimation of gestational age by measuring fetal dimensions 1
- Timing: Typically performed in the first or early second trimester 1
- Scope: Basic fetal biometry and confirmation of fetal viability 1
- Limited anatomical assessment: Does not include comprehensive evaluation of fetal structures 1
This basic scan establishes accurate dating, which is essential for all subsequent pregnancy management and screening tests 1.
Level 2 Ultrasound (Detailed Anatomical Scan)
Level 2 ultrasound provides comprehensive fetal structural evaluation 1:
- Primary purpose: Detailed examination of fetal anatomy to detect congenital abnormalities 1
- Standard timing: 18-20 weeks gestation, though can be performed from 11-14 weeks with modern equipment 1, 2
- Comprehensive assessment: Systematic evaluation of fetal skull, brain, spine, face, heart, abdomen, kidneys, bladder, limbs, and abdominal wall 3
- Detection capabilities: Identifies major structural abnormalities including neural tube defects, cardiac anomalies, abdominal wall defects, and skeletal dysplasias 1
Clinical Context and Performance
The detailed anatomical scan (Level 2) has specific detection rates that vary by timing 2, 3:
- First trimester (11-14 weeks): Detects approximately 93.3% of eight major anomalies with 99.99% specificity 2
- Cardiac anomalies: Detection rate of 55.8% in first trimester, improving with second trimester assessment 2
- Second trimester (18-22 weeks): Remains the standard for comprehensive anatomical screening, detecting 78.8% of major defects 3
Important Clinical Distinctions
High-risk pregnancies require Level 2 scans regardless of routine screening 1:
- Multiple gestations (especially monochorionic twins) 1
- Maternal obesity (scan should be performed at 20-22 weeks, potentially 2 weeks later than standard) 1
- History of bariatric surgery (monthly growth scans from viability recommended) 1
- Abnormal first trimester screening results 1
- Family history of congenital abnormalities 1
- Maternal medical conditions including diabetes or infections 1
Common Pitfall to Avoid
Do not assume a normal Level 1 scan excludes fetal abnormalities 3. Level 1 scans are not designed for anatomical assessment and will miss the majority of structural defects. A dedicated Level 2 anatomical scan at 18-20 weeks remains essential even after a normal first trimester dating scan 3, 4.
Additional Considerations
Level 2 scans may increase termination rates for major anomalies when performed early (RR 2.36 for second trimester detection leading to termination) 4. This allows parents informed decision-making but requires appropriate counseling about findings 4. The scans also reduce induction for post-maturity through accurate dating (RR 0.48) 4.