A pregnancy stress test monitors your baby’s heart rate to check whether the baby is doing well, and the results come down to a few key numbers. The most common version is the non-stress test (NST), which simply tracks the baby’s heartbeat during movement. A less common type, the contraction stress test, watches the heart rate during contractions. Both produce a paper tracing or digital readout that your provider interprets, but understanding the basics yourself can make those results far less confusing.
What the Test Actually Measures
During a non-stress test, two sensors are placed on your belly. One tracks the baby’s heart rate, and the other detects any uterine contractions. The test usually lasts 20 to 40 minutes, and the monitor produces a continuous tracing of the heart rate over time. Your provider is looking at three things on that tracing: the baseline heart rate, whether the heart rate speeds up when the baby moves, and whether there are any drops in heart rate.
Normal Baseline Heart Rate
A healthy fetal heart rate sits between 110 and 160 beats per minute. This is the “baseline,” the rate the heart settles into when the baby isn’t actively moving. If the baseline is consistently above or below that range, it can signal a problem, but brief fluctuations are completely normal. The baseline also has a natural waviness to it, called variability, which is a good sign that the baby’s nervous system is functioning well. A flat, unchanging line is more concerning than one that gently rises and falls.
Reactive vs. Non-Reactive Results
This is the main result you’ll hear after a non-stress test. A “reactive” result is the good one. It means the baby’s heart rate sped up at least twice within a 20-minute window. Each of those accelerations needs to rise at least 15 beats per minute above the baseline and last for at least 15 seconds. This is sometimes called the 15×15 rule, and it applies to babies at 32 weeks or later.
Those accelerations are a sign that the baby’s brain and heart are communicating properly. When a baby moves and the heart rate jumps in response, it means blood flow and oxygen levels are adequate. Think of it like checking that your baby is alert and responsive.
A “non-reactive” result means the monitor didn’t pick up enough of those accelerations within 40 minutes. This does not mean something is wrong. Babies sleep in cycles of 20 to 40 minutes, and if your baby happened to be in a deep sleep during the test, the heart rate may not show those characteristic spikes. Your provider may try gently stimulating the baby with sound or vibration, or simply extend the monitoring time to catch the baby in an awake period.
What Decelerations Mean
A deceleration is a temporary drop in the baby’s heart rate. These can appear on the tracing as dips below the baseline. Variable decelerations, which are brief and irregular drops, are actually quite common during heart rate monitoring and show up in as many as 50% of non-stress tests. They’re usually caused by temporary compression of the umbilical cord, often when the baby shifts position, and a single brief deceleration during an otherwise normal test is generally not a cause for concern.
Research on single sporadic decelerations during otherwise reactive tests found they were not associated with complications during labor, higher rates of cesarean delivery, or problems for the newborn. If your provider notes a deceleration but the rest of the tracing looks reassuring, it’s likely nothing to worry about. Repeated or prolonged decelerations are a different story and will prompt further evaluation.
How to Read a Contraction Stress Test
A contraction stress test is less routine and is typically ordered when a provider needs more information than the NST provided. During this test, mild contractions are triggered (either with medication or nipple stimulation), and the monitor watches how the baby’s heart rate responds to the temporary reduction in blood flow that happens during each contraction.
The results are framed as negative or positive, which can feel counterintuitive:
- Negative (normal): The baby’s heart rate does not slow down and stay slow after contractions. This is the reassuring result, suggesting the placenta is delivering oxygen well even under stress.
- Positive (abnormal): The baby’s heart rate drops after contractions and stays slow. This pattern, called late decelerations, can indicate the placenta isn’t keeping up with the baby’s oxygen needs during the stress of contractions.
A positive result doesn’t automatically mean an emergency, but it does mean your provider will likely move quickly toward additional evaluation or may discuss the timing of delivery depending on how far along you are.
What Happens After a Non-Reactive Result
If your non-stress test comes back non-reactive, your provider has several next steps available. The most common is a biophysical profile, which combines the heart rate monitoring of the NST with an ultrasound. The ultrasound adds four additional checks: the baby’s breathing movements, body movements, muscle tone, and the amount of amniotic fluid. Each component gets a score, and the total helps your provider build a more complete picture of the baby’s well-being.
Another option is a contraction stress test, described above, which assesses how the baby handles the reduced blood flow that comes with uterine contractions. In some cases, if the baby is at or near full term and results remain unclear, your provider may recommend inducing labor rather than continuing to monitor.
The key thing to understand about a non-reactive result is that it means “not enough information,” not “something is wrong.” Many non-reactive tests are followed by perfectly normal results on repeat testing or follow-up exams. The test is designed to be cautious, flagging any tracing that doesn’t clearly show a healthy pattern so it can be investigated further.
Why You Might Be Getting Stress Tests
Non-stress tests are most commonly used in higher-risk pregnancies. If you have gestational diabetes, high blood pressure, a baby measuring small for gestational age, low amniotic fluid, or if you’re past your due date, your provider may schedule regular NSTs, sometimes once or twice a week. They’re also used when you’ve noticed decreased fetal movement. The test is painless and noninvasive, which is part of why it’s the go-to screening tool for checking on the baby in the third trimester.
If you’re looking at your own test tracing, the main things to check are straightforward: a baseline heart rate in the 110 to 160 range, at least two heart rate accelerations of 15 beats lasting 15 seconds within 20 minutes, and no repeated or prolonged drops in heart rate. If those boxes are checked, the tracing is reactive and reassuring.