The fetal hippocampus. A possible pitfall and a useful anatomical landmark in evaluation of the biparietal diameter.

A well-demarcated hypoechoic region was seen just lateral to the collicular plate during measurement of the fetal biparietal diameter (BPD) in 101 (70%) of 144 ultrasound examinations and confirmed on postmortem sonographic water-bath studies of fetal brains. This structure was found to represent the hippocampus, with its lateral border formed by the trigone of the lateral ventricle. It should not be confused with an abnormal entity such as a ventricular cyst. Recognition of this hypoechoic region in conjunction with previously described structures is a useful method of obtaining consistent BPD measurements.