Baseline FHR variability has been quantitated into the mathematical indices of differential index (short-term variability) and interval index (long-term variability). Normal values have been determined. The effect that drugs used during labor have on FHR baseline variability has been evaluated. A clinically meaningful change in variability has been observed following low-dose administration of Demerol, morphine, Nisentil, Phenergan, and Vistaril. A statistically significant increase in variability has been observed following administration of magnesium sulfate.