This terminology was used more commonly prior to pregnancies being dated routinely by ultrasound.
If the gestation was unknown, a comparison of scan measurements of the fetal head and abdomen could give an indication as to whether the abdominal circumference was smaller than expected, with the head measurement (BPD or HC) acting as a proxy for gestational age.
Symmetrical growth restriction can be due to misdating, constitutional smallness, or very early onset FGR. Separate plotting of fetal head, abdomen and femur measurements will not distinguish between these causes.
Asymmetrical growth restriction is conventionally considered to be due to placental failure with `head sparing‘.
It is doubtful whether the distinction between symmetrical and asymmetrical growth failure adds any useful clinical information today. Further investigations are necessary for either, such as Doppler flow to assess placental function. If the growth deficit is severe, assessment for chromosomal abnormality still needs to be considered. The degree of deviation is more important than the symmetry between abdominal and head measurements
7. Gardosi J. Ultrasound biometry and Fetal Growth Restriction. Fetal and Maternal Medicine Review 2002; 13:4; 249-259
Cambridge University Press DOI: http://dx.doi.org/10.1017/S0965539502000426