The fetal radiation dose from 18F-FDG was estimated in a series of pregnant women who underwent a PET scan during a clinical workup for malignancies. Methods: Six pregnant patients were injected with 18F-FDG (activity range, 296-385 MBq). Three patients were scanned during the first trimester (1 with PET and 2 with PET/CT), 2 were scanned during the second trimester (with PET/MR imaging), and 1 was scanned during the third trimester (with PET). The time-integrated activity coefficients were derived from the fetal radioactivity concentrations measured on the images for all but 1 patient (in early pregnancy [5 wk]), in whom the activity in the uterus was used as a proxy. The coefficients of the mother's organs were derived from standard values (from the International Commission on Radiological Protection). Results: Fetal doses ranged from 6.29E-03 to 2.46E-02 mGy/MBq. An earlier bladder voiding reduced these doses by 25%-45%. The 2 patients who underwent PET/MR imaging-in whom fetal contours could be accurately delineated-displayed the lowest fetal absorbed dose, likely because of more accurate region drawing, with the inclusion of areas of both low and high fetal uptake. Moreover, PET/MR imaging did not necessitate additional radiation for attenuation correction. The placenta, delineated on a PET/MR imaging scan, concentrated 0.27% of the injected activity. Conclusion: Fetal radiation doses are higher in early pregnancy than in late pregnancy, and there can be considerable intersubject variability. However, the total absorbed dose is always well below the threshold for noncancer health effects throughout pregnancy. PET/MR imaging is the optimal PET procedure for imaging pregnant women because it is not associated with radiation for attenuation correction and allows more accurate dosimetric calculations.