Fetal heart rate monitoring is done either externally by a Doppler ultrasound monitor attached to the mother’s abdomen, or internally by elecrocardiography monitor attached to the fetus’ presenting part. Internal, or direct monitoring, requires rupture of membranes and cervical dilation of at least 1.5 cm.
A normal fetal heart rate tracing is characterized by:
- base line between 120-160 bpm
- beat-to beat variability (average 6-10 bpm)
- long term transient accelerations and decelerations with a frequency generally between 2-6 cycles/min
Variability is a reflection of the beat-to-beat adjustments of parasympathetic and sympathetic nervous system to a variety of stimuli.
Accelerations are an indication of fetal well being and are characterized by an amplitude of 10 beats/min from baseline and lasting no more than 10 seconds at a level 5 bpm above baseline.
The two most sensitive indicators of adequate cerebral oxygenation in the fetus are variability and accelerations. A normal baseline rate with moderate variability and accelerations is predictive of a well-oxygenated fetus.
Early decelerations correlate with uterine contractions and represent compression of the fetal head. Early decelerations start with the onset of contraction and end with contractions, with the peak deceleration correlating with the peak of uterine contraction. These are generally well tolerated by the fetus and are not worrisome tracings.