How to Read an NST Strip for Fetal Well-Being

A non-stress test (NST) is an non-invasive prenatal test used to assess fetal well-being during pregnancy. This test primarily evaluates the baby’s heart rate in response to its movements, providing information about the oxygen supply to the fetus. It is called “non-stress” because no additional stress is placed on the fetus during the procedure. Healthcare providers recommend an NST, particularly in the third trimester after 28 weeks of gestation, to ensure the baby is healthy before labor.

Decoding the NST Strip

An NST strip provides a visual representation of fetal heart rate and uterine activity over time. The paper moves at a standard speed, allowing for clear tracing of events. The strip features two main lines: the top line displays the fetal heart rate (FHR) in beats per minute (bpm), while the bottom line records uterine activity or contractions.

These tracings are obtained using external transducers placed on the mother’s abdomen. A Doppler ultrasound transducer monitors the fetal heart rate. A tocodynamometer measures external changes in the abdominal contour caused by uterine contractions.

Interpreting Fetal Heart Rate Patterns

Interpreting the fetal heart rate tracing involves evaluating several parameters to understand the baby’s condition. The baseline fetal heart rate is the average rate during a 10-minute window, excluding temporary accelerations or decelerations. A normal baseline ranges from 110 to 160 beats per minute, indicating a healthy functioning central nervous system and adequate oxygenation.

Variability refers to the irregular fluctuations in the fetal heart rate baseline, measured from peak to trough. Moderate variability, between 6 to 25 beats per minute, is a reassuring sign, reflecting a well-oxygenated fetus with a healthy neurological system. Minimal variability (5 bpm or less) or absent variability (no fluctuations) can suggest reduced oxygenation or a fetal sleep cycle, while marked variability (greater than 25 bpm) has unclear significance.

Accelerations are abrupt, temporary increases in the fetal heart rate above the baseline. For fetuses at or beyond 32 weeks gestation, an acceleration is defined as an increase of at least 15 bpm above the baseline, lasting for at least 15 seconds but less than two minutes. Before 32 weeks, an increase of 10 bpm for 10 seconds is considered an acceleration. These increases occur with fetal movement and indicate fetal well-being and adequate oxygen supply.

Decelerations are decreases in the fetal heart rate below the baseline, categorized by their appearance and timing relative to uterine contractions. Early decelerations are gradual decreases in FHR that mirror uterine contractions, beginning and ending around the same time as the contraction. These are benign and are associated with fetal head compression during contractions.

Variable decelerations are abrupt, V or W-shaped decreases in FHR that vary in their onset, depth, and duration relative to contractions. They are characterized by a drop of at least 15 bpm, lasting for at least 15 seconds but less than two minutes. These patterns suggest umbilical cord compression, which can temporarily reduce blood flow and oxygen to the baby.

Late decelerations are gradual decreases in FHR where the onset, nadir (lowest point), and recovery occur after the peak of a uterine contraction. These are more concerning, as they can indicate uteroplacental insufficiency, meaning reduced blood flow and oxygen exchange between the uterus and placenta.

Understanding Uterine Activity

The bottom tracing on the NST strip records uterine activity, indicating the presence and pattern of contractions. While the primary focus of an NST is the fetal heart rate response to movement, evaluating uterine activity provides additional context. External monitoring can show the frequency and duration of contractions.

Frequency is measured from the beginning of one contraction to the beginning of the next, indicating how often they occur. Duration is the length of time each contraction lasts, measured from its start to its end. While external monitoring can give a general sense of intensity by showing the relative strength of contractions, it does not precisely measure the actual pressure in millimeters of mercury (mmHg) like internal monitoring.

What a Reactive NST Means

A “reactive” NST result is a reassuring sign of fetal well-being, indicating the baby is receiving sufficient oxygen. To be classified as reactive, the NST needs to show a normal baseline fetal heart rate along with moderate variability.

Additionally, a reactive NST requires the presence of at least two accelerations within a 20-minute period. Each acceleration must meet specific criteria. If these criteria are not met within 20-40 minutes, the test is considered “non-reactive,” which prompts further evaluation but does not automatically mean there is a problem.

Next Steps After an NST

Following an NST, the healthcare provider will discuss the results and any necessary follow-up care. If the NST is reactive, no immediate additional actions are required, and routine prenatal care continues. The test may be repeated as part of ongoing surveillance, particularly in high-risk pregnancies or if the pregnancy extends beyond the due date.

If the NST result is non-reactive, it suggests that the fetus did not meet the expected heart rate acceleration criteria. This does not always indicate a problem; the baby might simply be in a sleep cycle or affected by maternal medications. In such cases, further evaluation is recommended, which may include a biophysical profile (BPP) combining an ultrasound with an NST, or a contraction stress test (CST) to assess the baby’s response to induced contractions.

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