FHR is proxy for fetal oxygenation - fetal brain modulates heart rate through autonomic nervous system

• External monitor - less invasive but less precise
• Internal monitor - require rupture of membranes with placement of fetal scalp electrode (FSE) and intrauterine pressure catheter (IUPC)

• Fetal heart rate
• Baseline is mean FHR rounded to 5 bpm during a 10-minute segment, for a minimum of 2 minutes
• Normal baseline between 110 and 160 bpm
• Variability: amplitude (peak-to-trough) of oscillation of FHR at baseline
• Absent - undetectable amplitude
• Minimal - ≤ 5 bpm
• Moderate (normal) - 6-25 bpm
• Marked - > 25 bpm
• Sinusoidal - cycle frequency of 3-5/min persisting ≥ 20 min
• Uterine activity (averaged over 30-minute window)
• Normal - ≥ 5 contractions in 10 minutes
• Tachysystole - > 5 contractions in 10 minutes (qualify as to +/- associated FHR decels)
• Acceleration - abrupt increase in FHR (onset to peak in < 30 s, lasting < 2 minutes in duration)
• a/w vigorous neonate, absence of acidosis
• Reactivity (reassuring NST) is 2 accelerations over 20 minutes; correlates to decreased risk of stillbirth within the week of the NST
• ≥ 32 wks EGA - peak of ≥ 15 bpm above baseline, duration ≥ 15s
• < 32 wks EGA - peak of ≥ 10 bpm above baseline, duration ≥ 10s
• Prolonged acceleration - duration ≥ 2 min but < 10 min
• Acceleration lasting ≥ 10 min = baseline change
• Deceleration - decrease in FHR
• Early, late, variable (see below)
• Prolonged deceleration - decrease in FHR ≥ 15 bpm, lasting ≥ 2 min and < 10 min
• Decel ≥ 10 min = baseline change

	Early	Late	Variable
FHR
Onset to nadir	≥ 30 s	≥ 30 s	< 30 s (abrupt) Decrease in FHR ≥ 15 bpm, lasting ≥ 15 s and < 2 min
Nadir	with peak of contraction	after peak of contraction	varies
Associated cause	Vagal response to fetal head compression - normal	Uteroplacental induced fetal hypoxia	Umbilical cord compression

Classification of FHR tracings