NIPT Cannot Be Performed at 24 Weeks for Structural Anomalies—Proceed Directly to Amniocentesis with Chromosomal Microarray
At 24 weeks with bilateral ventriculomegaly, a small cisterna magna, and a thin posterior nuchal fold, NIPT is not appropriate; you must proceed directly to diagnostic testing with amniocentesis and chromosomal microarray analysis. 1
Why NIPT Is Not the Right Test Here
Timing and Clinical Context Make NIPT Inappropriate
NIPT is a screening test designed for early pregnancy (after 10 weeks), not a diagnostic tool for structural anomalies identified at 24 weeks. 2 The patient is already past the window where screening would guide decision-making about diagnostic testing.
When structural abnormalities are detected on ultrasound—particularly multiple findings like bilateral ventriculomegaly with additional CNS anomalies—diagnostic testing is required, not screening. 1 NIPT cannot evaluate the full spectrum of genetic abnormalities associated with these structural findings.
The Society for Maternal-Fetal Medicine explicitly recommends that diagnostic testing (amniocentesis) with chromosomal microarray analysis should be offered when ventriculomegaly is detected. 1 This is a Grade 1B recommendation, meaning strong evidence supports this approach.
The Correct Diagnostic Pathway
Amniocentesis with Chromosomal Microarray Is the Standard of Care
Amniocentesis with chromosomal microarray analysis (CMA) is the definitive test for this clinical scenario. 1 CMA detects not only common aneuploidies but also copy number variants and microdeletions that NIPT would miss.
In cases of bilateral ventriculomegaly, CMA identifies clinically significant copy number variants in approximately 4.7% of cases beyond what conventional karyotyping detects. 3 This additional diagnostic yield is critical for counseling and management.
Conventional karyotyping alone is insufficient—CMA should be performed concurrently because it provides higher resolution and detects submicroscopic abnormalities. 3
Additional Essential Testing
Testing for cytomegalovirus (CMV) and toxoplasmosis should be performed when ventriculomegaly is detected, regardless of known exposure or symptoms. 1 This is a Grade 1B recommendation.
CMV DNA testing on amniotic fluid is recommended, as fetal infection is an important etiology of ventriculomegaly. 3 While the detection rate is relatively low (1.3% in one large series), the implications for prognosis are significant. 3
Fetal MRI should be considered to identify additional CNS abnormalities not visible on ultrasound, particularly when expert interpretation is available. 1 In complex ventriculomegaly (with other findings), MRI provides additional information in up to 78% of cases. 4
Why This Matters for Prognosis and Counseling
The Presence of Multiple Findings Changes Everything
Isolated mild ventriculomegaly (10-12 mm) has a favorable prognosis with >90% likelihood of normal neurodevelopment, but your case is NOT isolated—there are additional CNS findings (small cisterna magna, thin nuchal fold). 1
Non-isolated ventriculomegaly has significantly worse outcomes, with termination rates progressively increasing based on the severity and type of associated abnormalities. 3 The distinction between isolated and non-isolated disease fundamentally alters counseling.
Genetic abnormalities are found in 26.7% of complex ventriculomegaly cases, making diagnostic testing essential rather than optional. 4
Critical Pitfalls to Avoid
Do not order NIPT at this gestational age with structural anomalies present—it delays definitive diagnosis and provides no actionable information. 2 NIPT is a screening test that requires diagnostic confirmation when positive, and you already have ultrasound findings that mandate diagnostic testing.
Do not rely on prior negative NIPT results (if performed earlier) to exclude genetic abnormalities in the setting of new structural findings. 5 Mosaicism may have been undetected, and structural anomalies warrant direct fetal testing regardless of prior screening.
Do not skip CMV testing—it is specifically recommended for ventriculomegaly and can alter prognosis and management. 1
Do not delay amniocentesis waiting for "more information"—at 24 weeks, you are approaching the limits of gestational age where termination remains an option if severe abnormalities are confirmed, and families need time to process results and make informed decisions. 6