This study aims to investigate the complexity of developing fetal cortical surface using a 3D fast Fourier transform (FFT) fractal dimension (FD) estimator. Thirty-five fetal MR images were chosen at 22-36 weeks of gestational age (GA) including 30 normal brains and 5 testing brains. Observations of testing brains indicate that: cortical dysplasia (developmental delay) and twins have lower FD than that of normal fetal brains. Results of FD from normal brains show that are positively correlated to GA and evidently indicate cortical complexity increasing with developing weeks of GA, which are in good agreement with fetal brain development. It could be said that the 3D FFT FD estimator is a promising means for the quantification of complexity of the fetal cortical surface. (C) 2010 Elsevier Ltd. All rights reserved.