Objective: The aim of this study is to investigate the growing of the caecum and the vermiform appendix during the fetal period in human fetuses and the relation between growing and shape of the caecum and the localization of the vermiform appendix. Methods: 40 male and 40 female externally normal-looking fetuses were studied between 10 and 40 weeks of gestation and were subdivided into fetuses of the 1st, 2nd, and 3rd trimesters and full-term fetuses. The parameters of interest included covering of the caecum with peritoneum, shape of the caecum (long tube, symmetric saccule, asymmetric saccule, extremely large right side - atypical), axial length and width of the caecum localization of the vermiform appendix, length of vermiform appendix and length of the meso-appendix. Results: There was a difference in covering of the caecum with peritoneum between males and females; the caecum was predominantly tube shaped during the fetal period, with the asymmetric saccule towards the end of the fetal period; the localization of the vermiform appendix was subcaecal (39%) during the fetal period and postileal (34%) in female fetuses and subcaecal (48%) in male fetuses -the localization of the vermiform appendix changed according to the caecum shape during the fetal period; we found significant differences in the measurements of vermiform appendix, mese-appendix, and caecum among 1st-, 2nd-, and 3rd-trimester fetuses and full-term fetuses. Conclusions: There was an increase in the measurements for vermiform appendix, mese-appendix, and caecum with increasing gestational age; the localization of the vermiform appendix depended on the shape of the caecum, was different from that of adults and different between sexes, and there was also a significant difference in covering of the caecum with peritoneum between both sexes. Copyright (C) 2001 S. KargerAG, Basel.