The aim of the present study was to determine the dimensions of the mandible during the fetal period, the relationship between the growth rates of the angle of the mandible and the dimensions of the mandible. Furthermore the angle of union of the two halves of the body of the mandible on the horizontal mandibular plane, which was not described elsewhere, is explored in this study. One hundred and sixty-one human fetuses (83 males and 78 females) without any cranio-facial or mandibular asymmetry, external pathology or anomaly and aged between 9 and 40 weeks of gestation were used in the study. Cases were stratified into four groups according to their developmental ages, that is, first trimester, second trimester, third trimester, and term fetuses. Cranio-facial parameters in addition to bi-condylar, bi-gonial, bi-mental tubercular, condyle-gonion, gonion-mental tubercle, condyle-mental tubercle, gonion-pogonion distances were measured. The degree of the angle of the mandible, angle of union of the two halves of the body of the mandible and sagittal length of the base of the mandible were calculated. The means of the parameters with respect to gestational weeks and groups were computed. While there were no sex differences in any of the parameters (P > 0.05) there were very significant relationships between gestational age and parameters (P < 0.001). The mean degree of the angle of the mandible during the fetal period was 122 +/- 8 degrees. The mean a angle of the base of the mandible was 65 +/- 8 degrees. None of these angles varied significantly throughout the fetal period. Height of the ramus of the mandible increased more than the length of the body of the mandible and bi-gonial distance in the first and second trimesters while in the third trimester and term period they increased at the same rate. Comparisons of groups for the ratio of the transverse to the sagittal lengths of the mandible revealed significant differences between first and second trimesters with term group (P < 0.05). Accordingly, bi-gonial distance of the mandible increased more during the third trimester and term time than the first and second trimesters, compared with the sagittal length of the mandible. The morphometric features and architectural changes observed in the mandible do not totally occur during the fetal period but continue later in childhood and adulthood.