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372: Second trimester prenatal ultrasound for the detection of fetal structural anomalies and their associated risk for chromosomal abnormalities

      Objective

      To determine the association between specific structural anomalies found on second trimester ultrasound and the risk of chromosomal abnormalities.

      Study Design

      Prospective population-based cohort study of second trimester ultrasound among 8, 990 Californian women who were at increased risk of chromosome abnormality based on serum screening. The risk of chromosome abnormality, positive predictive values (PPV) were calculated when structural anomalies were identified among two groups of patients: those at high risk based on serum testing (directly observed in this study, where the risk of chromosome abnormality was 1/21) and women at low risk (modeled results) where the risk of chromosome abnormality was 1/1000.

      Results

      When a structural anomaly was diagnosed in the high risk population, the risk of the fetus having a chromosomal anomaly was high. Depending on the organ system where the anomaly was seen, the risk of the fetus having a chromosome abnormality varied from 43% for a GI anomaly, to 70% if a CNS anomaly was identified . However, even in the low risk population the risk of a fetus having a chromosome abnormality was high if a structural anomaly was seen. The risk ranged from 2.4% if a GI anomaly was seen, to 11.9% if a cystic hygroma was identified.
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      Conclusion

      Structural anomalies detected on second trimester ultrasounds are strongly associated with chromosomal abnormalities. Therefore, regardless of serum screen results, detection of structural anomalies should prompt the clinician to offer invasive testing as the risk of chromosome abnormality substantially outweighs the risk of fetal loss associated with amniocentesis.