BACKGROUND: In the present study we aimed to quantitatively evaluate the growth of the psoas major in human fetuses. MATERIALS AND METHODS: Using anatomical dissection, digital-image analysis (NIS Elements AR 3.0), volumetric hydrostatic method, and statistical analysis (Student's t-test, regression analysis), the 10 direct morphometric parameters (4 lengths, 2 widths, 3 projection surface areas, and volume) of the psoas major were evaluated, and then the 5 morphometric indexes (belly width-to-length ratio, tendon width-to-length ratio, belly-to-muscle projection surface area ratio, tendon-to-muscle projection surface area ratio, and tendon-to-belly projection surface area ratio) were calculated in 67 human fetuses of both sexes (31♂, 36♀) aged 16-28 weeks. RESULTS: Neither male-female nor right-left significant differences were found in relation to numerical data of the growing psoas major. Both the total muscle length and tendon length increased logarithmically, the belly length followed the third-degree polynomial function, both the maximal belly width and midway tendon width followed inverse functions, while the distance from the muscle origin to the widest part of the muscle belly, 3 (muscle, belly and tendon) projection surface areas, and volume increased commensurately to fetal age. CONCLUSIONS: In terms of morphometric parameters, the psoas major displays growth dynamics diverse to four functions: from a gradual inhibition of growth which is typical of both natural logarithmic functions (total length and tendon length) and a reverse model (belly width and tendon width) through a linear growth (distance between the origin and the widest belly level, muscle projection surface area, belly projection surface area, tendon projection surface area, and volume) to a hyperbolic growth (belly length).