The classification of small-for-gestational-age (SGA) fetuses into symmetrical and asymmetrical has been widely used since the introduction of ultrasound into obstetric diagnosis. The purpose of this study was to assess the possibility of relating body proportionality of the SGA fetuses to aetiological factors. Ultrasound measurements of the head circumference (HC), abdominal circumference (AC) and femur length (FL) were obtained in 348 SGA fetuses subdivided into aetiological groups (structural and/or chromosomal anomalies, twins, abnormal placental vascularization, maternal malnutrition, infectious diseases, drug addiction, unknown). The difference in Z-score between each couple of parameters (HC-AC, HC-FL, FL-AC) was used to assess the presence of asymmetry and its degree. The distribution of the Z-score differences was Gaussian; +1 S.D. and +2 S.D. were chosen as cut-off values of the Z-score for the definition of asymmetry. No significant differences in the frequency of HC-AC asymmetry could be detected among aetiological groups. SGA fetuses with abnormal placental vascularization and those with congenital malformations were more frequently asymmetric for the difference HC-FL and less frequently asymmetric for the difference FL-AC compared to the ones of unknown cause. It is concluded that body proportionality among SGA fetuses is a continuum, and that information about it derived from ultrasound measurements is of limited use in clinical practice.