The fetal heart rate (FHR) reflects a baby’s oxygen supply and overall well-being in the womb. Healthcare providers regularly monitor the FHR to ensure the fetus is responding appropriately. A sudden or sustained drop in the fetal heart rate is not a normal indicator that labor is about to begin. Instead, such a change before labor starts is a potential sign of distress requiring immediate medical attention.
Fetal Heart Rate Baselines and Normal Variations
The normal resting fetal heart rate (FHR) during the third trimester ranges from 110 to 160 beats per minute (bpm). This baseline rate is established when the fetus is not moving or being stimulated. A sustained rate below this range is called bradycardia, while a rate above it is known as tachycardia. Both can be signs of concern if they persist.
A healthy fetus exhibits temporary increases in heart rate called accelerations. An acceleration is defined as a rise of at least 15 bpm above the baseline, lasting for 15 seconds or more. These increases show that the baby’s central nervous system is functioning well and that the fetus is responding actively to movement or external stimuli.
Before the onset of labor, the baby’s heart rate remains stable within the normal baseline range, exhibiting these reassuring accelerations. A sustained decrease in FHR before labor is not a physiological marker of impending delivery. Any significant, persistent deviation from the expected baseline necessitates further investigation by a medical professional.
Understanding Fetal Heart Rate Monitoring
Healthcare providers use specific tools and tests to assess the fetal heart rate during late pregnancy. One common tool is the Non-Stress Test (NST), which measures the baby’s heart rate and movement over time. The goal of the NST is to identify at least two FHR accelerations within a 20-minute period, indicating a “reactive” result.
Monitoring can also be done as part of a Biophysical Profile (BPP). This combines the NST with an ultrasound assessment of indicators like fetal breathing, movement, tone, and amniotic fluid volume. These tests primarily use external monitoring, where a transducer is placed on the mother’s abdomen to record the heartbeat.
External methods are non-invasive and are used routinely during prenatal visits and initial labor assessments. Internal monitoring involves placing a thin wire electrode directly onto the baby’s scalp after the amniotic sac has ruptured. Internal monitoring provides a clearer, more accurate reading, which may be needed if the external monitor is not capturing a reliable tracing.
Fetal Heart Rate Changes During Active Labor
Once active labor begins, changes in the fetal heart rate often occur in response to uterine contractions. These temporary drops in heart rate are known as decelerations, and their pattern relative to the contraction determines their significance. The most common and generally benign change is the early deceleration.
Early decelerations are characterized by a gradual decrease and return to the FHR baseline that mirrors the uterine contraction. The lowest point of the heart rate drop occurs at the peak of the contraction. This pattern is believed to be caused by temporary compression of the baby’s head as it moves through the birth canal.
Head compression causes a reflex slowing of the heart rate through the vagus nerve. Because the drop in heart rate is brief and synchronized with the contraction, early decelerations are typically not associated with a lack of oxygen. They are viewed as a normal physiological response to the forces of labor and do not require specific intervention.
When Fetal Heart Rate Changes Indicate Concern
While early decelerations are usually reassuring, other patterns of FHR decrease indicate potential fetal distress and require immediate medical action.
Late Decelerations
One concerning pattern is the late deceleration, where the FHR drop begins after the contraction has peaked and recovers only after the contraction has ended. This delayed timing suggests a problem with blood flow to the placenta, often leading to insufficient oxygen supply for the fetus.
Variable Decelerations
Another concerning pattern is the variable deceleration, which appears as an abrupt, jagged, and irregular drop in the FHR that may or may not align with the timing of a contraction. Variable decelerations are most commonly caused by compression of the umbilical cord, which temporarily restricts blood flow. Frequent or severe variable decelerations signal that the baby is not tolerating the stress of labor well.
When late or severe variable decelerations occur, the medical team initiates intrauterine resuscitation measures to improve fetal oxygenation. These actions include repositioning the mother to relieve pressure on blood vessels, administering supplemental oxygen, and increasing intravenous fluids. If these interventions do not resolve the concerning FHR patterns, an expedited delivery, possibly via Cesarean section, may be necessary to ensure the baby’s safety.