The development of the brain vesicles between Carnegie stages (CS) 17 and 23 was analyzed morphometrically using 177 magnetic resonance image data derived from the Kyoto Collection of Human Embryos. Whole embryonic volume was 106.55 ± 21.08 mm3 at CS17, exponentially increasing to CS23 when it reached 1357.28 ± 392.20 mm3. Length of brain vesicles was 29.83 ± 2.52 mm at CS17, increased almost linearly and reached 49.31 ± 6.66 mm at CS23. The rate of increase was approximately 4.2 times higher on the dorsal side than on the ventral side. The increase in the length of the brain vesicles resulted mainly from that of the prosencephalon, and the rate of increase was three times higher on the dorsal side than on the ventral side of the prosencephalon.