Variability

The difference in the heart rate from beat to beat is called variability. It is caused by the effect of teh sympathetic and the parasympathetic nervous system. The sympethetic nervous system acts to increase the heart rate while the parasympethetic acts to decrease it. Normal variability is between 6-25 beats per min). Severe oxygen deprivation always depresses our nervous system, resulting in very little or absent variability. Other things that may cause reduced variability include fetal sleep cycles, cns depression, fetal anomalies or previous damage to the fetal brain, as well as a lower gestational age. Also, if tachycardia(high heart rate) is present, variability is reduced. Normal variability indicates the absence of severe hypoxia, but should be evaluated in context.

References:

Obstetrics: Normal and Problem Pregnancies, 5th ed. Gabbe et al.

Fanaroff and Martin's Neonatal-Perinatal Medicine, 8th ed. Martin et al.