Virtually continual exposure to 970-MHz microwaves in circularly-polarized waveguides was used to elicit fetal responses in Sprague-Dawley rats during gestation. A total of 250 rats were exposed to microwave radiation at whole-body averaged specific absorption rates (SAR) of 0.07, 2.4, or 4.8 W/kg or concurrently sham-irradiated for 22 h/day from the 1st through the 19th day of gestation. Prior to irradiation or sham irradiation, the rats were acclimated to cages in the waveguide system for 10 days and then bred. The rats were killed on the 20th day of gestation, and their fetuses counted and weighed, the fetal sternebrae ossifications counted, and the number of corpora lutea of the pregnancy determined. There were no alterations in pregnancy rates, fetal numbers (live, dead, resorbed, or total conceptuses), fetal weights, fetal skeletal maturation, or preimplantation losses after exposure at SARs of 0.07 or 2.4 W/kg. At SAR of 4.8 W/kg, only fetal body weight was significantly altered (- 12%, p = 0.012). Two of twelve rats died during the exposure at SAR of 4.8 W/kg. Bred, but non pregnant, rats that were exposed at SAR of 4.8 W/kg had significantly lower body weight gain than sham-irradiated rats; similar lower gain is assumed to have occured in the pregnant rats exposed at SAR of 4.8 W/kg, and whose fetuses were significantly smaller. We conclude that continual gestational exposure at SAR of 4.8 (but not 2.4 or lower) W/kg induces fetal alterations. Apparently, deleterious maternal effects are associated with these fetal changes. Although colonic temperature was not measured in these rats, it is expected that exposure at 4.8 W/kg was hyperthermal.