The Application of Crown-Chin Length to Crown-Rump Length Ratio in Predicting Fetal Skeletal Dysplasia at First Trimester.

Objective: To examine the feasibility of using crown-chin length (CCL) to crown-rump length (CRL) ratio in screening for skeletal dysplasia in the first trimester.

Methods: Four hundred and eighteen singleton pregnant women were recruited and the ratio of CCL to CRL was calculated according to gestational age. Fetuses with skeletal dysplasia were collected from database in the last 10 years. The CCL/CRL ratios were then calculated and the unpaired Student's t-test was to determine the significance of differences between normal fetuses and fetuses with skeletal dysplasia. Receiver operating characteristic curve was used to show the clinical sensitivity and specificity.

Results: In 418 normal fetuses, CCL increased linearly with gestation from a mean of 20 mm at 11 weeks to 37 mm at 13 weeks (CCL [mm] = 0.51-4 CRL, R  = 0.824, P = .000). There was also a significant linear association between fetal CCL/CRL ratio and CRL, from a mean of 0.48 at 11 weeks to 0.41 at 13 weeks (CCL/CRL = 0.63-3 CRL, R  = 0.108, P = .000). In 154 skeletal dysplasia cases, early pregnancy ultrasound images were available in only 16 cases. The CCL/CRL ratio in 10 of 16 fetuses with skeletal dysplasia was above the 95th percentile. Using the 95th percentile as a cut-off, the detection rate, specificity, false-positive rate, and positive likelihood ratio are 62.5, 72.6, 5, and 17.5%, respectively.

Conclusions: Increased fetal CCL/CRL ratio at 11-14 weeks' gestation is associated with an increased risk of skeletal dysplasia and may be useful in first-trimester screening for this condition.