Data from 354 embryos and fetuses between 20 and 200 mm crown rump length obtained by therapeutic abortion in 3 different countries were evaluated. All Danich and Hungarian and the majority of American specimens were measured immeditely after delivery in the fresh condition. In the mathematical evaluation linear regressions were calculated by the method of least squares for arbitrarily defined ranges to 20-50 mm and 50-200 mm crown rump lenghts. The material was analyzed statistically so that confidence limits could be drawn for the estimation of gestational age from crown rump length measurements. All data in the 20-50 mm range were combined, but beyond that fetal length the statistics for the Hungarian group were calculated separatley. The equation calculated to fit the data in the 20-50 mm rage is A = 46 + 0.71 L where A is gestational age in days and L is crown rump lenght in mm. The 95% confidence limits of regression are 0.57-0.83 days/mm and the correlation between gestational age and crown rump length is 0.65. Estimates of gestational age from sitting height measurements can be made + or - 15 days with 95% confidence. The equation for the combined Danish and American data in the range 50-200 mm is A = 64 + 0.41 L. The 95% confidence limits for the regression are 0.36-0.46 days mm and the correlation between gestational age and crown rump length is 0.70. Estimates of gestational age from crown rump length can be made + or - 26 days with 95% confidence. The data from the Hungarian study in the 50-200 mm sitting height range differ from those of the combined Danish and American material. The regression of days/mm (0.22) was significantly less at p. 01 level supporting the suspected bias in the Hungarian material, but the correlation between gestational age and crown rump length, 0.62, was not significantly less than that of the combined Danish and American data. Thus, if the difference in the slope was due to a bias, the bias was relatively consistent from patient to patient. Comparison of the results with those of Streeter (1920, 1951) indicates that the considerable discrepancy at the embryonic stages diminishes gradually in the fetal period and eventually becomes quite insignificant.