The development of biophysical monitoring of the fetus during labor and delivery is performed routinely.
- Electronic Fetal Monitoring.Intrapartum external electronic fetal heart rate (FHR) monitoring (ultrasound cardiography with transducers on mother's abdomen) is the most common obstetric procedure performed.
- Fetal well-being is assessed by uterine activity, baseline heart rate and variability and presence of accelerations, and periodic decelerations (Table 40-16: Assessment of Fetal Well-Being).
- During the active phase, contractions should occur every 2 to 3 minutes with peak intrauterine pressures of 50 to 80 mm Hg and resting pressures of 5 to 20 mm Hg.
- Baseline FHR ranges between 110 and 160 beats/min. Persistently elevated rates may be associated with chronic fetal distress. Abnormally slow rates may occur during the course of fetal hypoxia and acidosis.
- Baseline FHR variability ranges between 6 and 25 beats/min. Whereas normal baseline variability is a reassuring sign of normal fetal acid–base status, a smooth FRH tracing may reflect asphyxia. (Drugs also decrease variability.)
- Periodic FHR decelerations (early, late, variable) occur in association with uterine contractions (Fig. 40-2: Classification and mechanism of fetal heart rate patterns). Early decelerations typically accompany the onset of contractions. Late decelerations occur after the peak of uterine contractions and may reflect fetal hypoxemia. Variable decelerations are the most common and result from umbilical cord compression that results in activation of the carotid baroreceptor reflex. If late or variable decelerations are recurrent (at least half the contractions) or prolonged (15 beats/min or more below baseline lasting ≥2 minutes), there is a significant correlation with fetal acidosis, and delivery may be undertaken.
- The American College of Obstetricians and Gynecologists recommends a three-tiered system for the evaluation of fetal heart rate tracings (Table 40-17: Three-Tiered System for the Evaluation of Fetal Heart Rate Tracings).
- Ancillary Tests and Fetal Pulse Oximetry. Fetal scalp pH testing is become less frequent, and the routine use of fetal pulse oximetry is not currently endorsed.