The petrous portion of the temporal bone as shown on sonography between 14 and 40 weeks' gestation

The purpose of this study was to examine the feasibility of sonographically identifying, measuring, and constructing nomograms of the fetal petrous bone, the part of the temporal bone that houses the auditory apparatus.

A total of 248 consecutive pregnant women between 14 and 40 weeks' gestation were included. During a routine scan using transvaginal or transabdominal sonography, the fetal head was assessed in transverse sections. First, the parotid gland or external ear was identified, followed by the petrous bone. This annular echogenic structure, located medially and superior to the parotid gland or external ear, was measured at its largest anteroposterior diameter. The petrous bone on at least one side was measured; if both sides were visualized easily, both petrous bones were measured.

In 247 fetuses, at least one petrous bone was measured. The petrous bone measurement correlated well with the gestational age (R = 0.908; P < .001), biparietal diameter (R = 0.983; P < .001), and femur length (R = 0.913; P < .001).

Depiction of the petrous bone is feasible at all gestational ages and correlates well with the gestational age, biparietal diameter, and femur length. The usefulness of the ability to depict this bone in prenatal diagnosis of congenital malformations of the inner ear should be tested in prospective studies.