The relationship between the fetal TCD and the AC throughout pregnancy was investigated in a prospective, cross-sectional study of 700 well dated, normal pregnancies between 14 and 42 weeks of gestation and tested in 30 known SGA infants. Fetal measurements included the TCD, AC, BPD, HC, and FL. The TCD/AC ratio was calculated for each patient. Statistical analysis using linear regression and Student's t-test was performed. Strong correlation existed between gestational age and transcerebellar diameter (R2 = 0.9464), between gestational age and AC (R2 = 0.9685), and between TCD and AC (R2 = 0.9561). The TCD/AC ratio was normally distributed with a mean +/- SD of 13.69 +/- 0.94% (median, 13.71%). The 10th and 90th percentiles were 12.50% and 14.86%, respectively. The ratio remained constant throughout pregnancy with respect to gestational age (R2 = 0.0084). A TCD/AC ratio greater than the 90th percentile was present in 87% (26/30) known SGA infants. The TCD/AC ratio is a stable, gestational age-independent parameter that may be useful in the early detection of fetal growth abnormalities.