Three-dimensional sonographic description of the fetal frontal bones and metopic suture.

OBJECTIVE To describe the morphology of the frontal bones and metopic suture at 9-34 weeks of gestation using three-dimensional (3D) ultrasonography. METHODS This was a cross-sectional study of the frontal bones and metopic suture in 16 fetuses at 9-34 weeks of gestation. 3D ultrasonography was used to obtain volumes of the whole fetal skull in the mid-sagittal plane. The volumes were analyzed using the rendering mode and transparent maximum mode in order to achieve a clear view of the frontal bones and the metopic suture. RESULTS At 9 weeks, a small ossification center was visible in the middle of each supraorbital part of the frontal bones and by 11 weeks the frontal bones appeared as 'thick eyebrows'. In the second trimester there was progressive radial bone expansion and delineation of the metopic suture and in the third trimester there was closure of the metopic suture, starting from the glabella and moving upwards towards the anterior fontanelle. CONCLUSIONS This study provides 3D ultrasound images that illustrate the process of ossification of the frontal bones and the subsequent development of the metopic suture during prenatal life.