In this paper, we proposed a set of fetal brain MRI analysis methods to quantify the fetal brain tissue volume. We used deep learning-based brain mask extraction method to obtain brain mask and reconstructed 3D fetal brain volumes using registration-based reconstruction method. Then an age-specific atlas-based segmentation method was applied to segment three major tissues (White Matter, cortical Gray Matter, cerebrospinal fluid). The changes of intracranial volume and the three brain tissue volumes across different gestational ages were calculated and fitted with both linear and quadratic curves. The results demonstrated the effectiveness of the automatic 3D fetal MRI quantification methods.