Objective: The purpose of this study was to examine sonographic findings in fetuses with trisomy 13.

Methods: A retrospective review of the cytogenetic laboratory databases at 6 tertiary referral centers identified all cases of trisomy 13. The prenatal sonographic studies in fetuses of less than 22 weeks' gestation, done before invasive testing for karyotype, were reviewed for anatomic and biometric findings. We defined abnormal fetal biometric findings as a biometric measurement (biparietal diameter, abdominal circumference, or femur length) below the fifth percentile in the second trimester.

Results: There were 8 cases of trisomy 13 found in the first trimester and 54 cases found in the second trimester, a total of 62 in all. In the first trimester, 6 of 8 had an anomaly identified (4 with cystic hygroma). In the second trimester, 49 of 54 were identified by sonography; 45 had an anomaly, and 4 had an abnormal fetal biometric measurement without an anomaly. The 5 missed diagnoses had early gestational age (<17 weeks; n = 3) or an inadequate survey secondary to poor visualization. Overall, 22 of 54 fetuses with trisomy 13 had an abnormal biometric measurement. The most common anomalies detected in the second trimester were heart defects (n = 34), central nervous system anomalies (n = 30), facial clefts (n = 19), abnormal hands (n = 13), and genitourinary anomalies (n = 9).

Conclusions: Targeted sonography identified abnormal fetal anatomy or abnormal biometric measurements in 95% of fetuses with trisomy 13 in the second trimester after 17 weeks' gestation. A biometric measurement below the fifth percentile was noted in nearly half of cases in the second trimester.