OBJECTIVES: Cranial base development during fetal life is of major interest for many research purposes. In this study, we focused on fetal age estimations which are extremely important in forensic contexts. Nevertheless, there have been very few studies using occipital measurements to determine fetal age. OBJECTIVES: To evaluate change in the sagittal length/maximum width ratio of the basilar part of the occipital bone and to propose a simple and reliable method for fetal age determination. METHODS: A sample of 30 male and 7 female fetuses aged 24 to 41 weeks amenorrhea underwent CT scan. Occipital bones were reconstructed (Amira 4.0) and measurements were carried out on each part. RESULTS: A multivariate analysis (ANOVA) gave an age formula using right exoccipital measurements and a linear regression supplied the age of reversion of the sagittal length/maximum width ratio. CONCLUSION: This preliminary study clearly highlighted the promises of 3D CT-Scan studies of fetal cranial base and gave interesting results and the method deserves to be tested on a larger sample.