Fetal heart rate (FHR) monitoring is a standard procedure during labor, providing direct information about the baby’s condition as they navigate the stresses of the birthing process. Tracking the FHR assesses whether the baby is receiving sufficient oxygen, which is essential during contractions. The heart rate tracing acts as a real-time signal, indicating how the baby’s central nervous system and cardiovascular system are responding to the temporary decrease in blood flow that occurs with each uterine contraction. Evaluating the heart’s pace and rhythm helps medical professionals identify patterns that may suggest the need for intervention to ensure a safe delivery.
How Fetal Heart Rate is Monitored
The two main approaches to tracking the baby’s heart rate are intermittent auscultation and continuous electronic fetal monitoring (EFM). Intermittent auscultation involves periodically listening to the FHR, often using a handheld Doppler ultrasound device or a fetoscope, at regular intervals between and during contractions. This method allows for greater maternal mobility and is generally used for low-risk pregnancies.
Continuous EFM, common in many hospitals, involves placing two transducers on the mother’s abdomen, held in place with elastic belts. One external EFM transducer uses ultrasound to detect fetal heart movements and translate them into a tracing. The second device records the timing of uterine contractions. This non-invasive method can sometimes produce unclear tracings if the mother or baby moves significantly.
Internal monitoring is sometimes necessary when external monitoring is not providing a clear or reliable reading. This more precise method requires the amniotic sac to be ruptured and the cervix to be partially dilated. A thin wire, known as a fetal scalp electrode, is gently attached to the baby’s scalp to transmit the heart rate directly to the monitor. This provides a more accurate and consistent reading of the FHR, ensuring a clearer picture of the baby’s response to labor.
Understanding the Normal Baseline
The baseline FHR is the average rate of the baby’s heart over a 10-minute period, excluding temporary accelerations or decelerations. For a baby at full term, the normal baseline range is between 110 and 160 beats per minute (bpm). A heart rate consistently within this window suggests the baby is well-oxygenated and tolerating the labor process.
Deviations from this range signal changes in the baby’s state that warrant closer observation. A baseline FHR consistently above 160 bpm is defined as fetal tachycardia. Tachycardia can be an early sign of issues like maternal fever, infection, or a mild lack of oxygen, as the baby’s heart attempts to compensate.
Conversely, a baseline FHR consistently below 110 bpm is defined as fetal bradycardia. Bradycardia is a concerning sign, often indicating a serious reduction in oxygenation or a profound fetal response, and usually requires immediate medical attention. The degree and duration of these deviations help determine the urgency of the situation.
Interpreting Fetal Heart Rate Patterns
Beyond the baseline, the FHR tracing is interpreted by evaluating its fluctuations (variability) and its temporary changes (accelerations and decelerations). Moderate variability is a reassuring sign, appearing on the monitor as a slight, irregular jitteriness in the baseline heart rate, showing fluctuations of 6 to 25 bpm. This changeability indicates a responsive fetal nervous system that is well-oxygenated and able to adapt to stress.
Accelerations are temporary increases in the FHR, typically rising at least 15 bpm above the baseline and lasting for 15 seconds or more. These are a positive sign of fetal well-being and responsiveness, often occurring with fetal movement or during a contraction. The presence of accelerations suggests the baby is adequately oxygenated and coping with the birthing process.
Decelerations are temporary drops in the FHR that occur in response to contractions or other stimuli. They are categorized based on their appearance relative to the uterine contraction, providing clues about their cause. Early decelerations are gradual drops in heart rate that mirror the contraction; the lowest point of the deceleration occurs at the same time as the peak of the contraction. These are generally considered benign and result from compression of the baby’s head, which triggers a reflex slowing of the heart rate.
Variable decelerations are characterized by an abrupt, jagged drop and quick return to the baseline; they do not necessarily line up with the timing of the contraction. These are the most common type of deceleration and are often caused by temporary compression of the umbilical cord, briefly reducing blood flow. While they can be benign if infrequent and shallow, recurrent or severe variable decelerations can signal reduced oxygen reserve.
Late decelerations are the most concerning pattern, presenting as a gradual decrease in FHR that begins after the peak of the contraction and returns to the baseline only after the contraction has ended. This delayed timing indicates reduced blood flow from the placenta, suggesting the baby is not recovering quickly enough between contractions. Recurrent late decelerations are a strong signal of potential oxygen deprivation and require immediate attention.
When Fetal Heart Rate Requires Intervention
When the FHR tracing displays non-reassuring patterns, such as persistent late decelerations or a sustained low baseline, medical staff initiate corrective actions known as intrauterine resuscitation. The goal is to improve the baby’s oxygen supply and reverse the concerning heart rate pattern without immediate delivery. A common first step is to change the mother’s position, often to her side, to relieve pressure on major blood vessels and improve blood flow to the placenta.
Supplemental oxygen is frequently given to the mother via a mask to increase the oxygen concentration in her blood, which increases the amount available to the baby. Increasing intravenous fluids can raise maternal blood volume and improve placental perfusion. If the mother is receiving oxytocin to stimulate contractions, that medication is typically discontinued immediately to reduce the frequency or intensity of contractions, giving the baby more time to recover.
These interventions are designed to quickly resolve the non-reassuring tracing; nearly two-thirds of concerning patterns improve with these steps. If the abnormal heart rate patterns persist despite these measures, or if the tracing is considered ominous, the medical team will expedite delivery. This may involve an operative vaginal delivery using forceps or a vacuum, or, more often, an emergency cesarean section, to resolve potential oxygen deprivation.