Regarding maxillofacial morphogenesis there has been a long debate on the growth of the maxillary structure. Using 120 normal fetal maxillae of gestational ages from 16 to 41 weeks, palatal radiograms and frontal histologic sections were made. We have observed two pairs of accentuated growth areas in the fetal maxillae and named them primary growth centers to formulate the maxillary trapezoid (MT) by radiologic image. The MT is formed by four primary growth centers that are best demonstrated by palatal radiograms of the fetal maxilla as well as by frontal histologic sections. The dimensional increase in the MT during the fetal period is documented and statistically analyzed. From this series of results, we have suggested that the growth centers which demarcate the MT are the basic structures of the developing human maxilla. It was also found that the four primary growth centers are the most active sites for maxilla formation until 20 weeks of gestation and thereafter the growth of the maxilla is enhanced by the participation of the intramembranous bone formation along the periphery. This was in contrast to the central primary growth centers that have already finished maturation in the early fetal period and remain only as a peripherally radiating arrangement of thick trabecular bones.