STV

Fetal heart rate variability arises from the continuous push and pull of sympathetic and parasympathetic input on the sinoatrial node, and its presence is the single most reliable cardiotocographic indicator of an intact, well-oxygenated fetal central nervous system. Visual assessment on standard CTG uses moderate variability, defined as an amplitude range of six to twenty-five beats per minute, whereas computerized analysis such as the Dawes-Redman system measures short-term variation numerically, typically flagging values below about three milliseconds as concerning; this is used chiefly in monitoring severe early-onset fetal growth restriction, where it complements Doppler assessment of the umbilical and ductus venosus waveforms in deciding on delivery timing. Reduced variability is not specific to hypoxia: it also occurs with fetal sleep cycles lasting twenty to forty minutes, prematurity, maternal opioids, magnesium sulfate, corticosteroids, and fetal tachycardia. The user submission correctly identifies STV as short-term variation of the fetal heart rate. Absent variability with recurrent late decelerations is a category III tracing requiring urgent intervention.