Normal and abnormal development of the fetal anterior fontanelle: a three-dimensional ultrasound study

To describe the methodology for correct visualization of the anterior fontanelle using three-dimensional ultrasound, to report its normal development during gestation, and to compare this with abnormal development.

This was a cross-sectional prospective evaluation of development of the anterior fontanelle in a series of 78 normal fetuses and 47 fetuses with congenital anomalies between 12 and 38 weeks of gestation. The anterior fontanelle was visualized in a mid-sagittal view of the fetal head, preferably with a pocket of fluid between the fetal head and the uterine wall, to ensure an optimal acoustic window. Visualization using volume contrast imaging mode in the coronal plane (VCI-C) was preferred to static acquisition of three-dimensional (3D) volumes, as the former allows real-time evaluation of the 3D image. Anteroposterior and laterolateral diameters, perimeter and area of the fontanelle were measured offline, with the diameters normalized for biparietal diameter and the perimeter and area normalized for head circumference. The variables were then regressed against gestational age.

The best fitting regression model to describe the relationships between the fontanelle anteroposterior diameter and area and gestational age was a quadratic one, whereas a simple linear model fitted all remaining variables. All variables showed a positive or biphasic correlation with advancing gestational age, but a negative one after normalization for biparietal diameter or head circumference. Twenty of the 47 fetuses with abnormalities had abnormal (18 enlarged and two reduced) fontanelle dimensions, particularly those with chromosomal or non-chromosomal syndromes, primary or secondary cardiac overload, primary skeletal dysplasias or central nervous system malformations. Hydrops was not associated with abnormal fontanelle dimensions.

We have described the methodology to obtain correct visualization of the fetal anterior fontanelle. The actual size of the fontanelle increases during gestation, while its size in relation to the volume of the fetal head diminishes, possibly due to the rapid development of the brain hemispheres and the consequent outward growth of the calvarial bones. The fact that enlarged fontanelle dimensions may be associated with certain fetal abnormalities may be employed advantageously in the differential diagnosis of some syndromic conditions in utero.