The nonstress test (NST) is a common prenatal assessment used to evaluate the well-being of a developing fetus. This non-invasive procedure helps healthcare providers ensure the fetus is receiving adequate oxygen and responding appropriately to its environment, especially when there are specific concerns about fetal health.
Understanding the Nonstress Test
The nonstress test measures the fetal heart rate and its response to movement.
During the test, a pregnant individual lies comfortably, often in a reclined chair. Two elastic belts are placed around the abdomen; one monitors the fetal heart rate, and the other measures uterine contractions.
The information is recorded on a monitor. The test typically lasts 20 to 30 minutes, extending to 40 minutes if the fetus is in a sleep cycle.
Interpreting a Reactive Result
A “reactive” nonstress test result indicates fetal health and adequate oxygenation, signifying the fetal heart rate accelerates appropriately in response to movement.
For a test to be considered reactive, specific criteria must be met: two or more fetal heart rate accelerations within a 20-minute period.
Each acceleration needs to increase the heart rate by at least 15 beats per minute (bpm) above the baseline rate and last for a minimum of 15 seconds.
These criteria apply to pregnancies beyond 32 weeks of gestation. For pregnancies at or below 32 weeks, the acceleration criteria are an increase of 10 bpm for at least 10 seconds.
The presence of these accelerations suggests a normally functioning central nervous and cardiovascular system in the fetus.
When an NST is Performed
A nonstress test is performed in the third trimester of pregnancy, after 28 weeks, when the fetal heart rate begins to respond consistently to movements.
This test is not routine for all pregnancies but is often recommended when there are specific concerns or risk factors.
Common reasons include being past the due date (beyond 40 weeks), high-risk pregnancies due to maternal conditions like diabetes or high blood pressure, and other chronic medical conditions.
Other situations that may prompt an NST include reduced fetal movement, concerns about fetal growth (small for gestational age), or multiple pregnancies.
A history of complications in previous pregnancies or certain blood incompatibilities, such as Rh negativity, can also lead to an NST being ordered.
What Happens After a Reactive Test
Following a reactive nonstress test, results are typically shared with the pregnant individual shortly after the procedure.
A reactive outcome usually provides reassurance about the fetus’s well-being and indicates no immediate further action is required.
In most cases, a reactive test means the pregnancy can continue with routine prenatal care.
The need for subsequent nonstress tests depends on the initial reason for the test and the healthcare provider’s assessment of ongoing risk factors.
If the clinical situation remains stable, repeat testing may not be necessary. However, if underlying conditions persist, the healthcare provider may schedule future NSTs to continue monitoring fetal health.