Transverse cerebellar diameter has been described as a tool to check for gestational age, but there are no available studies that attempt to determine differences in transverse cerebellar diameter between groups of different ethnic origin. The present study was undertaken to check for differences in ultrasound-measured transverse cerebellar diameter between pregnant women of autochthonous Belgian origin and migrant women from Morocco and Turkey.A prospective cross-sectional study was performed. Patients were pregnant women presenting between 17 and 40 weeks of gestational age. Only uncomplicated singleton pregnancies with a known date of the last menstrual period confirmed by first trimester ultrasound were included. The father of the child had to be of the same ethnic origin as the mother. Polynomial regression was fitted for the three different ethnic groups.The transverse cerebellar diameter was obtained in 471 singleton fetuses, including 333 Belgian, 69 Moroccan and 69 Turkish. F-tests on the residual sums of squares of different fits demonstrated significant effects of ethnicity on the regression of the measurement versus gestational age (P < 0.00005). In a third order polynomial regression model the second order coefficient was significantly higher and the third order coefficient was significantly lower for the Moroccan group.The transverse cerebellar diameter is not independent of the ethnic origin of the patient. When using the transverse cerebellar diameter for the evaluation of fetal growth or for dating a pregnancy, care should be taken to rely on charts appropriate for the ethnic group, as demonstrated here for Moroccan fetuses.