During routine ultrasound examination of 178 normal pregnant women at 17-24 weeks, several biometric measurements were performed including transverse cerebellar diameter, cavum septum pellucidum, anterior and posterior horns of cerebral ventricles and nuchal soft tissue. The transverse cerebellar diameter is easy to obtain at this gestational age even in occiput posterior position and seems to be independent of head shape and inter-individual constitutional discrepancies. Transverse cerebellar diameter seems to be a good marker for gestational age compared to other clinical and biometric parameters.