Accurate determination of gestational age is essential for adequate pregnancy monitoring. In this scenario, fetal MRI appears as a complementary exam to ultrasound. This article presents the use of basic digital image processing techniques, applied to fetal images obtained through magnetic resonance imaging, to extract the diameter of fetal brains, estimating gestational age. The strategy developed in this work can extract the characteristics of the image, providing information on identifying the gestational age in the analyzed image, comparing with the measurements described in the literature.