A fetal heart tracing is a visual recording of your baby’s heart rate over time, printed alongside a record of uterine contractions. Reading one means evaluating five key features: baseline heart rate, variability, accelerations, decelerations, and contraction patterns. Each feature tells you something different about how the baby is tolerating labor, and together they paint a picture of fetal well-being.
What You’re Looking At on the Strip
A standard fetal monitor produces two continuous lines on a paper strip or digital screen. The top tracing shows the fetal heart rate in beats per minute (bpm), with the vertical axis typically scaled from 30 to 240 bpm. The bottom tracing shows uterine activity, recording the timing, duration, and relative strength of contractions. Reading a tracing means looking at both lines together, because the relationship between contractions and heart rate changes is where the most important information lives.
The horizontal axis represents time. Each small square equals 10 seconds, and each large block equals one minute. This time scale matters when you’re measuring how long a heart rate change lasts or counting how many contractions occur in a given window.
Baseline Fetal Heart Rate
The baseline is the average heart rate when the baby is not accelerating, decelerating, or moving. A normal baseline falls between 110 and 160 bpm. You determine it by looking at the heart rate over a 10-minute window, excluding any temporary rises or dips, and identifying the rate the tracing keeps returning to.
A baseline above 160 bpm is called tachycardia. Common causes include maternal fever, infection, dehydration, or certain medications. A baseline below 110 bpm is bradycardia, which can result from prolonged cord compression or, in some cases, is simply normal for a particular baby late in pregnancy. A baseline that drifts gradually upward or downward over the course of labor is worth noting, because trending changes can be more informative than any single reading.
Variability: The Most Important Feature
Variability refers to the small, irregular fluctuations in the heart rate from beat to beat. On the tracing, it looks like a jagged, squiggly line rather than a smooth one. This feature reflects a healthy, functioning nervous system and is widely considered the single most reassuring element of a fetal heart tracing.
Variability is classified by the amplitude of those fluctuations:
- Absent variability: The tracing looks flat, with no visible fluctuations. This is concerning and may indicate the baby’s brain is not getting enough oxygen.
- Minimal variability: Fluctuations are detectable but very small, no more than 5 bpm in amplitude. This can be normal if the baby is in a sleep cycle (which typically lasts 20 to 40 minutes) or if the mother has received pain medication. It becomes concerning when it persists.
- Moderate variability: Fluctuations range from 6 to 25 bpm. This is the ideal pattern. It strongly suggests the baby is well-oxygenated.
- Marked variability: Fluctuations exceed 25 bpm. The significance is less clear, but it can sometimes indicate the baby is being stimulated or is in an early stage of oxygen deprivation.
One pattern to recognize is the sinusoidal pattern, a smooth, undulating, wave-like tracing that looks almost like a sine wave. It lacks the normal beat-to-beat irregularity. True sinusoidal patterns are rare and are associated with serious fetal anemia or significant distress. Research published in the American Journal of Obstetrics and Gynecology found that nearly all true sinusoidal tracings were linked to significant fetal or neonatal complications. This pattern looks different from the normal jagged variability and should not be confused with a baby who is simply having regular accelerations.
Accelerations: A Good Sign
Accelerations are temporary increases in the heart rate above the baseline. They’re a reassuring sign that the baby’s nervous system is responding normally, often triggered by fetal movement. On the tracing, they appear as upward bumps.
The criteria for what counts as an acceleration depend on gestational age. For babies at 32 weeks or later, the heart rate must rise at least 15 bpm above baseline and last at least 15 seconds (but less than 2 minutes). For babies younger than 32 weeks, the threshold is lower: a rise of at least 10 bpm lasting at least 10 seconds. The presence of accelerations, especially alongside moderate variability, is one of the most reassuring combinations you can see on a tracing.
Decelerations: Timing Is Everything
Decelerations are temporary drops in heart rate below the baseline. They are the feature that requires the most careful interpretation, because their significance depends almost entirely on when they occur relative to contractions. This is where reading both lines of the tracing together becomes essential.
Early Decelerations
These dips mirror contractions almost perfectly. The heart rate begins to drop as the contraction starts, reaches its lowest point at the peak of the contraction, and recovers as the contraction ends. They are thought to result from pressure on the baby’s head during contractions, which triggers a reflex slowing of the heart. Early decelerations are considered benign. Because the heart rate starts recovering immediately after the contraction peaks, there is very unlikely to be any oxygen deprivation involved.
Late Decelerations
Late decelerations look similar in shape to early ones, but the timing is shifted. The heart rate drop begins 20 to 30 seconds after the contraction starts, and the lowest point occurs after the contraction has already peaked. This lag time is the critical clue. It suggests the placenta is not delivering oxygen efficiently during contractions, because hypoxia during a contraction takes time to develop. Even shallow late decelerations (drops of just 10 to 15 bpm) are concerning when they recur with every contraction, particularly if variability is also diminished.
Variable Decelerations
Variable decelerations have no consistent timing relationship to contractions. They can start before, during, or after a contraction, and they tend to look abrupt and angular on the tracing, dropping sharply and recovering sharply, sometimes resembling a “V” or “W” shape. They are typically caused by compression of the umbilical cord. Occasional, brief variable decelerations with a quick return to baseline and preserved variability are common and usually not concerning. Repetitive deep variables, or those that are slow to recover, warrant closer attention.
The key to differentiating deceleration types is focusing on timing rather than shape. A deceleration that starts recovering as soon as the contraction peaks points to a benign, mechanical cause. One that doesn’t begin recovering until well after the contraction has passed suggests an oxygen-related problem with a built-in delay.
Uterine Activity on the Bottom Tracing
The lower portion of the strip records contractions. With external monitoring, you can see the timing and duration of contractions but not their true strength, since the sensor measures abdominal wall tension rather than intrauterine pressure. With internal monitoring using a pressure catheter placed inside the uterus, you get actual pressure readings.
Normal labor involves up to five contractions in a 10-minute window, averaged over 30 minutes. More than five contractions in 10 minutes is called tachysystole. This matters because the baby receives fresh oxygenated blood between contractions. When contractions come too frequently, the baby doesn’t get enough recovery time, which can lead to heart rate changes on the upper tracing.
How External and Internal Monitoring Differ
External monitoring uses two belts placed around the abdomen. One belt holds a Doppler ultrasound sensor that detects the fetal heart rate, and the other holds a pressure sensor that registers contractions. This method is noninvasive and works in most situations, but the signal quality can be affected by the baby’s position or maternal body type.
Internal monitoring uses a small wire electrode placed on the baby’s scalp to record the heart rate directly, and sometimes an intrauterine pressure catheter to measure contraction strength. This gives a much cleaner, more accurate tracing. However, it can only be used after the membranes have ruptured (your water has broken) and the cervix has begun to dilate. There is a small increase in infection risk with internal devices. The choice between methods depends on how labor is progressing and whether the external tracing is giving a reliable signal.
Putting It All Together
When you look at a fetal heart tracing, work through each feature systematically. Start by identifying the baseline rate and checking whether it falls in the normal range. Then assess variability: is the line jagged and irregular (good) or smooth and flat (concerning)? Look for accelerations as reassuring signs. Then examine any decelerations carefully, comparing their timing to the contraction pattern on the bottom tracing. Finally, check the contraction frequency to make sure the uterus is not contracting too often.
The most reassuring tracing has a baseline between 110 and 160 bpm, moderate variability, the presence of accelerations, no late or significant variable decelerations, and normal contraction frequency. The most concerning tracings combine absent variability with recurrent late decelerations or bradycardia, suggesting the baby is not getting adequate oxygen.
Most tracings in labor fall somewhere in between, with features that are neither perfectly reassuring nor immediately alarming. Interpretation is about recognizing patterns over time rather than reacting to a single moment. A tracing with moderate variability and occasional variable decelerations tells a very different story than one with minimal variability and repetitive late decelerations, even if both show heart rate dips. Context, trending, and the combination of features matter far more than any single element in isolation.