A nonstress test (NST) monitor prints two lines on a strip of paper. The top line tracks your baby’s heart rate, and the bottom line tracks uterine activity, including contractions. Understanding what each line shows, and what the key patterns mean, can help you follow along during your appointment instead of staring at squiggly lines wondering if everything looks normal.
What the Two Lines Show
The monitor uses two sensors strapped to your belly. One is an ultrasound sensor placed over your baby’s chest area, which picks up the fetal heart rate and displays it as the top tracing on the strip. The other is a pressure sensor called a tocodynamometer, positioned near the top of your uterus, which detects tightening. It shows up as the bottom tracing.
The paper scrolls slowly, usually at 3 centimeters per minute, so time moves from left to right. The vertical axis on the top tracing is scaled in beats per minute (bpm), typically ranging from about 60 to 210. The vertical axis on the bottom tracing shows relative pressure. The entire recording usually lasts 20 to 40 minutes.
Reading the Baseline Heart Rate
The first thing to look at on the top tracing is the baseline, which is the average heart rate when the baby is at rest (not moving or reacting to anything). A normal baseline falls between 110 and 160 bpm. Most healthy babies hover somewhere in the 130 to 150 range during the third trimester.
A baseline above 160 bpm is called tachycardia, and a baseline below 110 bpm is called bradycardia. Either can happen temporarily for harmless reasons, like the baby just being active, but a persistently abnormal baseline gets flagged for closer evaluation.
What Variability Looks Like
If you look closely at the top tracing, the line isn’t smooth. It has a jagged, squiggly quality. That’s variability, and it’s one of the most important things your provider looks at. It reflects the constant push and pull between the baby’s nervous system speeding up and slowing down the heart, which is a sign of a healthy, well-oxygenated brain.
Variability is measured by the size of those tiny fluctuations, peak to trough:
- Absent: The line looks flat, with no visible fluctuation.
- Minimal: Fluctuations are detectable but less than 5 bpm in range.
- Moderate (normal): Fluctuations between 6 and 25 bpm. This is what your provider wants to see.
- Marked: Fluctuations greater than 25 bpm, which can sometimes indicate the baby is under stress.
On the strip, moderate variability looks like a healthy, irregular zigzag. A flat or nearly flat line is the pattern that raises concern.
Accelerations: The Key to a “Reactive” Result
Accelerations are temporary spikes in the baby’s heart rate, and they’re what the whole test is really looking for. When a baby moves, stretches, or kicks, the heart rate should jump up briefly. On the strip, an acceleration looks like a hill or bump rising above the baseline.
For babies 32 weeks or older, 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. Before 32 weeks, the threshold is lower: 10 bpm for 10 seconds counts, because younger babies produce smaller accelerations.
A “reactive” NST, which is a normal result, requires at least two qualifying accelerations within a 20-minute window. If your baby is sleeping and not moving, those accelerations won’t appear, and the test may need to run longer. Fetal sleep cycles can last 20 to 40 minutes, which is exactly why some tests extend to 40 minutes before anyone draws conclusions. Your provider may also try to wake the baby with a gentle buzzer or vibrating device placed against your belly.
Decelerations: Dips Below the Baseline
Decelerations are drops in the heart rate and appear as valleys or dips on the top tracing. They’re classified by their shape and timing relative to contractions on the bottom tracing.
- Early decelerations mirror the contraction almost perfectly. The heart rate dips gradually as the contraction peaks and recovers as it ends. These are generally considered benign and often result from pressure on the baby’s head.
- Late decelerations look similar in shape but start after the contraction peak and don’t recover until after the contraction ends. The delay is the distinguishing feature. Late decelerations can suggest the placenta isn’t delivering oxygen efficiently during contractions.
- Variable decelerations are sharp, sudden drops that can happen at any point relative to a contraction. They often look like a “V” or “W” shape. These are usually caused by brief compression of the umbilical cord and are common. Occasional, brief variable decelerations are not automatically concerning, but repeated or deep ones get more attention.
During a standard NST for a healthy pregnancy, your provider wants to see no late or variable decelerations. The absence of these patterns, combined with a normal baseline and good accelerations, makes up a reassuring tracing.
Reading the Bottom Tracing
The lower line on the strip comes from the tocodynamometer, and it measures the tightening of your uterine muscle. It’s good at detecting when contractions happen and how frequently they come, but the external sensor cannot measure how strong they actually are. The height of the bumps on this tracing shows relative changes in pressure against the sensor, not absolute force.
A contraction appears as a rounded hill that rises from the resting baseline and then returns. Even mild Braxton-Hicks contractions you might not feel can show up here. Your provider will look at the timing of these bumps in relation to any heart rate changes on the top tracing, since the relationship between contractions and the baby’s heart rate is what reveals whether the baby is tolerating uterine activity well.
You may also notice small blips on the bottom tracing that don’t look like full contractions. These can be fetal movements. Some monitors have a button you can press each time you feel the baby kick, which marks the strip so your provider can see whether heart rate accelerations line up with movement.
What a Normal Result Looks Like
Putting it all together, a reassuring NST strip has a baseline heart rate between 110 and 160 bpm, moderate variability (that healthy zigzag pattern), at least two accelerations of 15 bpm lasting 15 seconds within 20 minutes, and no concerning decelerations. This is called a reactive, or Category I, tracing.
A non-reactive result means the test didn’t capture enough accelerations in the allotted time. This doesn’t automatically mean something is wrong. The most common reason is simply that the baby was asleep. If sound stimulation doesn’t produce a reactive pattern, the next step is often a biophysical profile, which combines an ultrasound with the NST data to evaluate the baby’s breathing movements, muscle tone, and amniotic fluid level. That test produces a score from 0 to 10, with 8 to 10 considered normal.
Getting a Good Tracing
Your position during the test affects the quality of the recording and how quickly the baby responds. Lying flat on your back can compress major blood vessels, reducing blood flow to the uterus and potentially making the baby less active. Research shows that lying on your left side or sitting in a reclined position produces better results and achieves reactivity faster. The left lateral position in particular has been identified as the most efficient position for completing the test quickly. Most clinics will position you in a recline or on your side, but if you’re placed flat on your back and feel lightheaded or the test is taking a long time, asking to shift to your left side is reasonable.
Eating a snack before the test can also help, since a small rise in blood sugar tends to increase fetal activity. Some providers recommend this, though it’s not a formal requirement.