A fetal monitor strip is a continuous printout (or screen display) with two tracings stacked on top of each other. The top tracing shows the baby’s heart rate, and the bottom tracing shows the mother’s uterine contractions. Learning to read both together is the key to understanding what’s happening during labor, because the real information comes from how the baby’s heart responds to each contraction.
What the Two Tracings Show
The strip is printed on grid paper that scrolls at a standard speed, usually 3 centimeters per minute in the United States. Time runs left to right. Each small square on the horizontal axis represents 10 seconds, and each bold vertical line marks one minute.
The top tracing records the fetal heart rate (FHR) in beats per minute. The vertical scale typically runs from about 30 bpm at the bottom to 240 bpm at the top. The bottom tracing records uterine activity. When an external monitor (called a tocodynamometer) is used, it picks up the timing and frequency of contractions but cannot precisely measure their strength. An internal pressure catheter, which requires the membranes to be ruptured, is considered the gold standard because it measures contraction intensity in actual pressure units (mmHg) along with frequency and duration.
Step 1: Find the Baseline Heart Rate
The baseline is the average heart rate the baby maintains between contractions and between any temporary rises or dips. You determine it by looking at a 10-minute window and identifying the rate the tracing hovers around most of the time, excluding accelerations and decelerations. A normal baseline falls between 110 and 160 bpm, the range used by most major obstetric guidelines worldwide. A large analysis of nearly 79,000 tracings found that 120 to 160 bpm best captures the statistically normal range.
A baseline above 160 bpm is called tachycardia. A baseline below 110 bpm is called bradycardia. Both can have benign explanations (a sleeping baby may drift toward the lower end, and maternal fever can push the rate higher), but sustained shifts outside the normal range deserve attention.
Step 2: Assess Variability
Variability refers to the small, irregular fluctuations in the heart rate from one beat to the next. On the strip, it looks like a jagged, squiggly line rather than a smooth one. You assess it by measuring the difference between the highest and lowest points of those tiny fluctuations (the peak-to-trough amplitude) within a given segment.
- Absent variability: The line looks flat and smooth, with no detectable fluctuation.
- Minimal variability: Fluctuations are visible but the amplitude is 5 bpm or less.
- Moderate variability (normal): Amplitude ranges from 6 to 25 bpm. This is the most reassuring finding on any strip, because it suggests the baby’s nervous system is healthy and responsive.
- Marked variability: Amplitude exceeds 25 bpm. The tracing looks wildly jagged.
Moderate variability is the single most important sign of fetal well-being. Even when other parts of the tracing look concerning, the presence of moderate variability is generally reassuring. Absent or minimal variability that persists, on the other hand, raises concern, especially when paired with other abnormal features. Keep in mind that babies cycle through sleep and wake states, so a short stretch of minimal variability (20 to 40 minutes) can be perfectly normal if the baby is sleeping. Certain medications, particularly corticosteroids given to help lung development in preterm babies, can also temporarily reduce variability.
Step 3: Look for Accelerations
Accelerations are abrupt jumps in heart rate above the baseline. They look like upward spikes on the strip. For a baby at 32 weeks or beyond, a normal acceleration rises at least 15 bpm above the baseline and lasts at least 15 seconds. This is sometimes called the “15 by 15” rule. For younger fetuses (under 32 weeks), the threshold is lower: 10 bpm for 10 seconds.
Accelerations are a good sign. They typically happen when the baby moves and indicate a responsive, well-oxygenated nervous system. Two or more accelerations within a 20-minute window is the hallmark of a “reactive” tracing on a non-stress test.
Step 4: Identify Decelerations
Decelerations are temporary drops in heart rate below the baseline. This is where reading the strip gets more nuanced, because the shape, timing, and cause of each dip differ. There are three main types, and understanding each one depends on looking at the bottom tracing (contractions) at the same time.
Early Decelerations
These are gradual, shallow dips that mirror the contraction almost perfectly. The lowest point of the heart rate dip lines up with the peak of the contraction, creating a symmetrical U-shape. Early decelerations are caused by pressure on the baby’s head during a contraction and are considered benign. They often appear during active labor as the baby descends into the birth canal.
Late Decelerations
Late decelerations look similar to early ones in shape (gradual onset, gradual return), but the timing is shifted. The lowest point of the dip comes after the peak of the contraction. If you were to draw a vertical line at the contraction’s peak, the heart rate nadir would fall to the right of it. This delay is the defining feature. Late decelerations suggest the placenta is not delivering enough oxygen to the baby during contractions, a condition called uteroplacental insufficiency. A single late deceleration can be insignificant, but repetitive lates, especially with reduced variability, are a concerning pattern.
Variable Decelerations
Variable decelerations are the most common type and the easiest to spot because they look dramatic. They are abrupt, sharp drops (the onset to the lowest point takes less than 30 seconds) of at least 15 bpm lasting at least 15 seconds but less than 2 minutes. Their shape and depth vary from one contraction to the next, which is how they got their name. They are primarily caused by compression of the umbilical cord. Occasional, brief variables with a quick return to baseline and good variability are usually not worrying. Variables that become deeper, longer, or slower to recover deserve closer attention.
Step 5: Read the Contraction Pattern
The bottom tracing tells you how often contractions are coming and roughly how long each one lasts. Contraction frequency is counted as the number of contractions within a 10-minute window, averaged over 30 minutes. During active labor, three to five contractions per 10 minutes is typical. In the second stage (pushing), frequency can reach five to six per 10 minutes, and contraction pressure typically increases from about 25 to 50 mmHg in early labor up to 80 to 100 mmHg during pushing.
If you’re watching an external monitor, you’ll see the contractions as bell-shaped humps rising from a flat resting line. The height of those humps on an external monitor does not reliably reflect how strong the contractions are, because the sensor’s readings are affected by the mother’s body type and the belt position. What you can reliably measure externally is timing: how far apart the peaks are and how long each hump lasts.
Putting It All Together
Clinicians in the United States use a three-tier system to categorize the overall tracing.
Category I (normal): Baseline between 110 and 160 bpm, moderate variability, no late or variable decelerations, and accelerations may or may not be present. This is a reassuring strip that requires no intervention.
Category II (indeterminate): Everything that doesn’t fit neatly into Category I or Category III. This is a large, catch-all group. Examples include minimal variability without decelerations, variable decelerations with a slow return to baseline, or a single prolonged deceleration. Most strips during labor fall into Category II at some point. These tracings require continued monitoring and sometimes corrective steps like repositioning the mother, giving fluids, or adjusting any medication that stimulates contractions.
Category III (abnormal): Either absent variability with recurrent late decelerations, recurrent variable decelerations, or bradycardia. A sinusoidal pattern also falls here. The sinusoidal pattern is distinctive: a smooth, undulating wave that looks like a sine curve, with no beat-to-beat variability. It is associated with serious fetal conditions, most often severe anemia. True sinusoidal patterns are rare but are treated as emergencies.
Common Factors That Change the Strip
Several things can alter what you see on the tracing without necessarily meaning the baby is in trouble. A baby in a quiet sleep cycle will show reduced variability and fewer accelerations for stretches of 20 to 40 minutes before “waking up” and returning to a more active pattern. Maternal fever raises the baby’s baseline heart rate. Corticosteroids, commonly given between 24 and 34 weeks of pregnancy to accelerate fetal lung maturity, can significantly affect variability for up to several days. Magnesium sulfate, used for seizure prevention in preeclampsia or for fetal neuroprotection in very preterm deliveries, does not appear to significantly change fetal heart rate parameters based on available evidence, though it can make the mother feel sluggish and affect her own heart rate.
The baby’s position also matters practically. If the baby shifts, the external sensor can lose the signal entirely, producing gaps or erratic-looking jumps on the strip. This is an artifact, not a real heart rate change, and usually resolves when a nurse repositions the belt.