A baby’s heart rate, or Fetal Heart Rate (FHR), measures the number of times the fetal heart beats per minute. Monitoring FHR assesses the baby’s well-being and oxygen supply before and during labor. A decrease in the baby’s resting heart rate is not a reliable indicator that labor is imminent. The FHR changes healthcare providers track are immediate responses to the environment inside the womb and the stress of contractions, not predictors of when labor will begin. FHR monitoring ensures the fetus is tolerating the conditions of pregnancy and labor.
Understanding Normal Fetal Heart Rate and Variability
The normal baseline FHR for a fetus in the third trimester ranges between 110 and 160 beats per minute (bpm). This rate is established by averaging heartbeats over a ten-minute period, excluding temporary accelerations or decelerations. The baseline rate decreases slightly as pregnancy progresses due to the maturation of the parasympathetic nervous system, but it remains within this range.
A significant aspect of a healthy heart tracing is “variability,” which refers to minor, irregular fluctuations in the FHR from beat to beat. This variability results from the interaction between the sympathetic and parasympathetic nervous systems, indicating an intact and well-oxygenated central nervous system. Moderate variability is a reassuring sign, characterized by fluctuations between 6 and 25 bpm. Fluctuations outside this range, such as minimal or absent variability, can suggest the fetus is under stress or not receiving adequate oxygen.
Fetal Heart Rate Monitoring: When and Why it is Used
Fetal heart rate monitoring is used during late pregnancy and continuously during labor to assess the baby’s health. The goal of this monitoring is to detect potential issues with fetal oxygenation, which can be life-saving if identified promptly. Monitoring is frequently used in high-risk pregnancies, such as those involving maternal diabetes or preeclampsia, or when there are concerns about fetal growth.
One common method for assessment before labor is the Non-Stress Test (NST), which measures FHR in response to fetal movements. A reactive result shows a healthy increase in heart rate with movement, confirming adequate oxygen supply. During labor, continuous electronic monitoring is employed using an external device on the mother’s abdomen or sometimes an internal electrode attached to the baby’s scalp. This continuous tracing allows providers to correlate the baby’s heart rate patterns directly with uterine contractions.
FHR Changes: What They Indicate During Labor and Delivery
Significant changes in FHR are observed during labor, in direct response to the mechanical and physiological stress of uterine contractions. These changes are categorized as either accelerations or decelerations and are evaluated in the context of the contraction pattern. An acceleration is a temporary increase in the FHR, usually by at least 15 bpm for at least 15 seconds, and is considered a positive sign of fetal well-being and adequate oxygenation.
Decelerations, which are temporary drops in the heart rate, are classified based on their shape and timing relative to the contractions. Early decelerations are gradual decreases that mirror the contraction, with the lowest point (nadir) occurring at the contraction’s peak. These are considered benign, often caused by compression of the fetal head in the birth canal, which triggers a reflex slowing of the heart.
Late decelerations are also gradual but are delayed, beginning after the contraction has started and reaching their nadir after the contraction’s peak. They are a pattern of concern because they can indicate utero-placental insufficiency, meaning the baby is not recovering quickly enough from the temporary reduction in blood flow caused by the contraction. Variable decelerations are abrupt and jagged, occurring at variable times in relation to the contraction, and are most often associated with umbilical cord compression. These FHR shifts are a direct result of the birthing process, confirming that a decrease in heart rate is a sign of response to labor rather than a signal for its beginning.