Trisomy 18 Detection on 20-Week Anatomy Scan
No, trisomy 18 is not always detectable on a routine 20-week anatomy ultrasound, though the detection rate is very high at 94-100% when a complete anatomic survey is performed. 1, 2
Detection Rates and Key Findings
The sensitivity of detecting trisomy 18 on second-trimester ultrasound ranges from 94% to 100% in experienced hands, but this depends critically on obtaining complete fetal visualization. 1, 2, 3
What Makes Detection Possible
When trisomy 18 is present, multiple structural anomalies are almost always evident on ultrasound, not just isolated soft markers 4:
- Cardiac defects are present in 55-84% of cases 1, 2, 3
- Central nervous system abnormalities occur in 34-87% of cases 1, 2, 3
- Upper extremity/hand anomalies (particularly clenched hands) are seen in 53-95% 1, 2, 3
- Growth restriction is present in 39-51% 1, 2
- Lower extremity/foot deformities (talipes) occur in 63% 3
All fetuses with trisomy 18 will have 4 or more sonographic abnormalities when completely visualized (median of 8 abnormalities per examination), and 87% will have two or more abnormalities detected 3, 5.
Critical Caveat: Technical Limitations
The 3-6% of cases that are initially "missed" are almost exclusively due to incomplete fetal visualization from technical constraints, not because trisomy 18 can present with normal anatomy. 2
- In one study, all 7 fetuses (14%) with no initial anomalies had incomplete anatomic surveys due to technical factors 2
- When these same fetuses were rescanned approximately 2 weeks later with complete visualization, abnormalities were detected in all cases 2
- Detection relies entirely on completing a thorough fetal anatomic survey 2
Important Clinical Context
Isolated soft markers alone (like isolated choroid plexus cysts) are NOT how trisomy 18 presents 4, 2:
- While choroid plexus cysts are present in 30-50% of trisomy 18 fetuses, no fetuses with trisomy 18 have been found with isolated choroid plexus cysts 4, 2
- When trisomy 18 is present, multiple major structural anomalies will be evident, including structural heart defects, clenched hands, talipes deformity, fetal growth restriction, and polyhydramnios 4
Practical Implications
If a 20-week anatomy scan shows completely normal fetal anatomy with adequate visualization of all structures, trisomy 18 is effectively ruled out. 1, 2, 3 However, if technical factors limit complete visualization (maternal body habitus, fetal position, oligohydramnios), a repeat scan should be performed to ensure adequate assessment 2.
Biochemical screening alone is inadequate - second-trimester biochemical screening has only a 2% positive predictive value for trisomy 18, while ultrasound detection of fetal anomalies is the most effective screening test 5.