Fetal Crown-Chin Length to Crown-Rump Length Ratio as a Prenatal Sonographic Marker for Triploidy at First Trimester

To investigate the ratio of Crown to Chin length (CCL) to Crown-rump length (CRL) between triploid and normal fetuses at first trimester and establish a reference range of fetal CCL/CRL ratio.

Three hundred and twenty-five normal and 12 triploid fetuses were reviewed in this study. The image of fetal Crown-rump length (CRL) was acquired retrospectively. CCL and CRL were measured offline by two experienced sonographers, we obtained each averaged value of CCL and CRL as the final data for analysis. A normal range of CCL was established and CCL/CRL ratio was analyzed in normal and triploid fetuses.

In 325 normal fetuses, CCL increased with gestational age following a linear trend from 20 mm at a CRL of 45-36 mm at a CRL of 84 mm (CCL (mm) = 3.65 + 0.38 CRL, R²  = 0.821, P = .000). The CCL/CRL ratio decreased with gestational age from a mean of 0.46 at a CRL of 45 mm to 0.41 at a CRL of 84 mm (PML/CRL = 0.502-0.001 CRL, R²  = 0.093, P = .000). All 12 triploid fetuses had a CCL/CRL ratio above the 95th percentile. When the 95th percentile are used as cutoff values, the detection rate, false-positive rate, and the positive likelihood ratio are 100%, 5%, and 20, respectively.

The present study demonstrates that fetal CCL/CRL ratio can be used and effective ultrasound marker in screening for triploidy during the first trimester.