In forensic medicine, specialists might face difficulties when estimating age at death from fetal remains. Depending on the state of preservation, this age assessment is essentially based on the diaphyseal size of long bones. In a previous work, for the measurement of fetal femoral ossified shafts, we already established a simple and reliable method using a radiographic protocol. Since we previously stated that radiographic measurement values were closer to real anatomical size than ultrasonographic ones, in the present study we decided to check the importance of the difference between radiographic and anatomical measurements. Therefore, we dissected 30 pairs of fetal femurs and compared the difference between the two kinds of measurements (in percentages). This difference seemed to be slight (4.027%), but it was large enough to entail significant differences (p < 0.001). In order to provide a correction factor for radiographic measurements, we established a linear regression formula, which was tested on another sample of 30 pairs of dissected femurs. As a consequence of the good results, we improved the linear regression using a powerful statistical tool: the bootstrap. Finally, we obtained a simple equation that allowed us to figure out the real anatomical size with an R2 of 99% and a mean relative difference of 0.153% (with a standard error of 0.252 mm, and therefore a 95% confidence interval with limits of -0.35 and 0.657 mm). This difference did not entail any significant differences (p = 0.498), and therefore, we concluded that with the proposed correction, radiographic measurements can easily be used by forensic specialists in their daily tasks or to establish new growth standards in order to best fit their population of interest.