Fetuses with Down syndrome have an enlarged anterior fontanelle in the second trimester of pregnancy.

OBJECTIVE: Neonates with Down syndrome are known to have an enlarged anterior fontanelle. The aim of this study was to assess whether fetuses diagnosed with Down syndrome in the second trimester have larger anterior fontanelles in comparison with normal euploid fetuses.
METHODS: The study population included 13 fetuses with trisomy 21 and 26 normal euploid fetuses analyzed between the 19(th) and the 23(rd) weeks of gestation. The anterior fontanelle was assessed by three-dimensional ultrasound, with the midsagittal plane of the fetal head being the reference view for acquisition of the volume. Anteroposterior and laterolateral diameters, perimeter and area of the fontanelle were then measured offline; the diameters were normalized for biparietal diameter and the perimeter and area were normalized for head circumference. Non-parametric statistical analysis was used to compare the mean values of all variables in the two groups of fetuses. Intra- and interobserver variability were also assessed.
RESULTS: All variables except the fontanelle laterolateral diameter were significantly greater in Down syndrome fetuses than in controls, with anterior fontanelle perimeter/head circumference and fontanelle area/head circumference ratios showing the highest sensitivity for the detection of Down syndrome. Using a cut-off of 2.1 for the fontanelle area/head circumference ratio, the sensitivity and specificity for the detection of Down syndrome were 77% and 96%, respectively.
CONCLUSION: During the mid-trimester the dimensions of the anterior fontanelle are significantly increased in fetuses with Down syndrome in comparison with normal euploid fetuses. This finding may be of help in the detection of trisomy 21 at the time of the anomaly scan.