Fetal Distress

The theory of fetal distress is that inadequate oxygen (asphyxia) during labour causes brain damage and death in babies, and that abnormal patterns in fetal heart rate preceed those adverse outcomes. According to that theory, it is thought that monitoring the FHR will allow for timely intervention to prevent those adverse outcomes.

However, Henci Goer makes the following points about the theory of fetal distress:

• The baby’s heart rate during labor correlates poorly with measures of the baby’s condition at birth.
• Measures of the baby’s condition at birth correlate poorly with long-term outcomes.
• Comparisons of EFM and intermittent listening show that EFM offers no long-term benefits in either low or high-risk pregnancies (although it may have short-term benefits especially when oxytocin is used during labour).
• Comparisons of EFM and intermittent listening show that EFM increases the likelihood of cesarean and vaginal instrumental delivery, infection and cerebral palsy in premature babies.
• The "cure" for fetal distress - caesarean or assisted (forceps/vacuum) delivery, can itself be asphyxiating or traumatising.
• Infant neurologic morbidity rarely correlates with labour events.

The theory of fetal distress is so well accepted that care providers accept it as a likely 'given' in childbirth, requiring that routine fetal heart rate monitoring is done, when in fact it is of next to no benefit to mother and baby. Fetal heart rate monitoring intervenes with the flow of labour and can cause undue distress, stress and concern to the mother should the results not be all rosy.

Fetal Heart Rates

The usual heart rate of babies during labour is between 120 to 160 beats a minute. Normal physiological responses to the birthing involve drops in FHR during contractions. The peak low point of the baby's heart rate coincides with the peak of a contraction, then goes back up as the contraction declines.

Physiological changes in heart rate are also seen in response to pressure on the baby's head by pelvic floor muscles, or the pelvis itself as the baby passes through it. As the baby is passing through the pelvis, the bony structure can also make it hard to pick up a fetal heart rate which may make some mothers panic upon discovering they can't hear the heart beat.

Fetal heart rate can drop as low as 60 beats per minute without danger to the baby, but this should not happen for too many contractions in a row. A reasonable drop is around 90 beats per minute.

Other Resources

"Obstetric Myths versus Research Realities" - Henci Goer

Interpretation of the fetal heart rate during labour

"Fetal Tachycardia
Fetal tachycardia is defined as a baseline heart rate greater than 160 bpm and is considered a nonreassuring pattern (Figure 3). Tachycardia is considered mild when the heart rate is 160 to 180 bpm and severe when greater than 180 bpm. Tachycardia greater than 200 bpm is usually due to fetal tachyarrhythmia (Figure 4) or congenital anomalies rather than hypoxia alone.16 Causes of fetal tachycardia are listed in Table 5.

Persistent tachycardia greater than 180 bpm, especially when it occurs in conjunction with maternal fever, suggests chorioamnionitis. Fetal tachycardia may be a sign of increased fetal stress when it persists for 10 minutes or longer, but it is usually not associated with severe fetal distress unless decreased variability or another abnormality is present.4,11,17

Fetal Bradycardia
Fetal bradycardia is defined as a baseline heart rate less than 120 bpm. Bradycardia in the range of 100 to 120 bpm with normal variability is not associated with fetal acidosis. Bradycardia of this degree is common in post-date gestations and in fetuses with occiput posterior or transverse presentations.16 Bradycardia less than 100 bpm occurs in fetuses with congenital heart abnormalities or myocardial conduction defects, such as those occurring in conjunction with maternal collagen vascular disease.16 Moderate bradycardia of 80 to 100 bpm is a nonreassuring pattern. Severe prolonged bradycardia of less than 80 bpm that lasts for three minutes or longer is an ominous finding indicating severe hypoxia and is often a terminal event.4,11,16 Causes of prolonged severe bradycardia are listed in Table 6.

TABLE 6: Causes of Severe Fetal Bradycardia
Prolonged cord compression
Cord prolapse
Tetanic uterine contractions
Paracervical block
Epidural and spinal anesthesia
Maternal seizures
Rapid descent
Vigorous vaginal examination"

Interesting to note that prolonged cord compression and cord prolapse are rare in physiological childbirth. Tetanic contractions are ones without a break like the ones that occur when labour is induced or augumented - something that does not happen in a freebirth, likewise same goes for epidural and spinal anesthesia and paracervical block.

Monitoring Fetal Heart Rate at Gentlebirth Archives