A non-stress test (NST) is a common prenatal assessment used to monitor a developing baby’s well-being. This non-invasive procedure evaluates the baby’s heart rate patterns in response to its movements. Understanding specific patterns, such as decelerations, is key to interpreting these tests.
What is a Non-Stress Test?
A non-stress test (NST) involves placing two belts around a pregnant person’s abdomen: one monitors the baby’s heart rate, the other tracks uterine contractions. Called “non-stress” because it avoids inducing contractions or external stress, the test observes heart rate changes over 20 to 40 minutes to assess reactivity and overall health. A normal, or “reassuring,” NST shows a baseline fetal heart rate typically ranging between 110 and 160 beats per minute. During the test, the baby’s heart rate is expected to show “accelerations,” temporary increases in heart rate. These accelerations occur in response to fetal movement, indicating a healthy, oxygenated baby. An NST is considered reactive if there are at least two accelerations within a 20-minute period.
Visualizing Decelerations
Decelerations on an NST tracing appear as dips in the fetal heart rate below baseline. Their appearance and timing relative to uterine contractions classify them into different types. Each type has a distinct visual pattern, providing clues about the baby’s condition.
Early decelerations are a gradual decrease in fetal heart rate that mirrors the shape and timing of a uterine contraction. The lowest point, or nadir, occurs at the same time as the contraction’s peak. The heart rate returns to baseline as the contraction ends, creating a symmetrical, “U” shaped dip aligning perfectly with the contraction wave.
Late decelerations also appear as a gradual decrease in fetal heart rate, but they are delayed in onset and recovery relative to the uterine contraction. The heart rate drop begins after the contraction has peaked, and it does not return to baseline until after the contraction has ended. This creates a “U” shaped dip shifted to the right, occurring after the contraction.
Variable decelerations are distinguished by their abrupt and often jagged appearance, resembling a “V,” “W,” or “U” shape. These decelerations are a sudden drop in heart rate, usually by at least 15 beats per minute, lasting for at least 15 seconds but less than two minutes. Their timing relative to contractions can vary, appearing before, during, or after a contraction, or even spontaneously.
Prolonged decelerations are a decrease in fetal heart rate of at least 15 beats per minute below the baseline, lasting for two minutes or more but less than ten minutes. These drops are significant and sustained. If the heart rate drop lasts for ten minutes or longer, it is considered a baseline change rather than a deceleration.
Interpreting Decelerations
Interpreting decelerations depends on their specific visual characteristics and the overall NST tracing context. Each type suggests different underlying physiological events, helping healthcare providers assess the baby’s well-being and determine appropriate next steps.
Early decelerations are generally considered benign and are often associated with head compression during labor. As the baby’s head is compressed, it stimulates the vagus nerve, temporarily slowing the heart rate. This type is typically not a cause for concern and does not indicate fetal distress or a lack of oxygen.
Late decelerations are more concerning as they often suggest uteroplacental insufficiency, meaning reduced blood flow or oxygen transfer from the placenta to the baby. This can be due to various factors affecting the placenta’s ability to deliver oxygen efficiently. Persistent late decelerations can indicate the baby is not tolerating the uterine environment well and may be experiencing oxygen deprivation.
Variable decelerations are commonly linked to umbilical cord compression, which temporarily reduces blood flow. Their abrupt and irregular nature reflects the sudden compression and release of the cord. While isolated variable decelerations can be common and often benign, recurrent or severe ones may suggest significant cord compression impacting the baby’s oxygen supply.
Prolonged decelerations indicate a more sustained interruption in the baby’s oxygen supply and are always a cause for immediate evaluation. These can be triggered by various events, such as prolonged cord compression, maternal hypotension, or uterine hyperstimulation. Their presence signals a need for prompt assessment and intervention to identify and correct the underlying cause.
Clinical Responses to Decelerations
When decelerations are identified during an NST, healthcare professionals respond based on their type and severity. The goal is to optimize the baby’s oxygenation and determine if further intervention is necessary to improve fetal well-being and ensure a safe outcome.
For early decelerations, no specific intervention is typically needed because they are considered a normal physiological response. Healthcare providers will continue to monitor the tracing to ensure no other concerning patterns develop. The focus remains on routine observation of the baby’s heart rate.
In cases of late or significant variable decelerations, several immediate steps may be taken. Repositioning the mother, often to her left side, can improve blood flow to the placenta and relieve potential cord compression. Administering supplemental oxygen to the mother may increase the oxygen available to the baby. Intravenous fluid administration can improve maternal blood volume and placental perfusion.
If these initial measures do not resolve the decelerations, or if the patterns are severe and recurrent, further evaluation is necessary. This might include a more comprehensive assessment of the mother’s hydration status or a re-evaluation of medication effects. Continuous monitoring of the fetal heart rate tracing is essential to observe the baby’s response to interventions.
In situations where decelerations persist or worsen, indicating potential fetal distress, healthcare providers may consider more urgent interventions. This could range from preparing for an expedited delivery if the baby’s condition does not improve. The specific course of action is determined by the overall clinical picture, including the baby’s gestational age and the mother’s health status.