To test the hypothesis that a narrow intensity band of geomagnetic activity contributes to Sudden Infant Death, 32 pregnant rats were exposed for two to three days before expected parturition either to a coil that generated 0.5 Hz sine-wave, 5 to 10 nanoTesla magnetic fields, or to a reference coil (<1 nT) in the same room. The field was off for 30 min every 4 h during the exposure. The orientation of the coils was perpendicular in space and activated alternately in four blocks of experiments. The litters born to the exposed mothers contained significantly fewer pups (M = 14.1,SD= 2.1) than those exposed to the control conditions (M = 16.2, SD = 2.7). There were significantly fewer numbers of males and fewer numbers of females in litters exposed to the fields generated in the east-west and north-south directions, respectively. These results support the hypothesis that a specific temporal configuration of brief periods of geomagnetic activity can produce an increased incidence of nonvital fetuses, neonates, or infants.