This article discusses issues regarding administration of radiopharmaceuticals to pregnant women or nursing mothers. Uncertainties in calculated dose estimates and possible biological effects on the unborn child are presented. Models and dose estimates for pregnant women at several stages of gestation are given; the radionuclide of highest concern is (131)I-NaI due to its affinity for the fetal thyroid and the potentially high fetal thyroid doses. The article also reviews the extant literature regarding the expression of radiopharmaceuticals in breast milk, and suggested time periods for interruption of breast feeding after a nursing mother receives a radiopharmaceutical, if needed. Again, (131)I-NaI is often the radiopharmaceutical of most concern, for the same reasons in the nursing infant as were shown for the unborn child. Strategies for preventing unwanted administrations of radiopharmaceuticals to these patients are reviewed, with strategies for minimizing radiation doses where possible.