A non-stress test (NST) is a common prenatal screening that monitors an unborn baby’s heart rate. Its primary goal is to assess fetal well-being by observing how the baby’s heart rate responds to movements. This assessment helps healthcare providers gain insight into the baby’s oxygen supply and overall health, typically performed in the third trimester of pregnancy, usually after 28 weeks gestation.
Interpreting NST Results
A “reassuring” or “reactive” NST indicates that the baby is likely well-oxygenated and healthy. This outcome is characterized by the fetal heart rate increasing appropriately in response to movement. A reactive NST shows at least two accelerations of the fetal heart rate within a 20-minute period, each increasing the heart rate by at least 15 beats per minute above baseline and lasting for a minimum of 15 seconds. This pattern suggests adequate oxygenation and a responsive fetal nervous system.
In contrast, a “non-reassuring” or “non-reactive” NST occurs when these expected heart rate accelerations do not meet the criteria, such as fewer than two accelerations, or those that are too weak or too short. A non-reactive result might also involve a lack of variability in the heart rate or the presence of decelerations, where the heart rate temporarily drops.
A non-reassuring NST does not automatically indicate a severe problem with the baby. Instead, it signifies that further investigation is necessary. The test has a high false positive rate.
Common Causes for Non-Reassuring Results
Several factors can lead to a non-reassuring non-stress test result, many of which are temporary and not indicative of a serious health concern. A frequent reason is simply that the baby is asleep during the test. Fetal sleep cycles can last for 20 to 40 minutes, during which time movement and heart rate accelerations may be minimal or absent.
The baby’s gestational age can also influence the results. Preterm babies might not show reactive patterns due to their developing nervous systems. Maternal factors can also play a role; certain medications, such as sedatives or beta-blockers, can depress the fetal heart rate and activity. Maternal dehydration or low blood sugar could also affect fetal activity during the test.
While less common, a non-reassuring NST can sometimes point to more significant underlying issues. These include concerns with the placenta’s function or umbilical cord compression. Fetal distress, characterized by sustained abnormal heart rate patterns, is another possibility that warrants further evaluation.
Subsequent Actions After a Non-Reassuring NST
When an NST yields a non-reassuring result, healthcare providers typically take several initial steps to encourage fetal activity and re-evaluate the baby’s heart rate. Often, attempts are made to wake the baby. This might involve using a small buzzer-like device placed on the mother’s abdomen, which emits a sound to stimulate the baby. The mother might also be encouraged to drink juice or eat a snack to increase the baby’s activity, or to change her position.
If the NST remains non-reassuring after these initial interventions or if there are other concerns, additional tests are usually performed for a more comprehensive assessment. A common follow-up is a Biophysical Profile (BPP), which combines an ultrasound with the NST. The BPP evaluates several aspects of fetal well-being, including fetal breathing movements, body movements, muscle tone, and the volume of amniotic fluid.
Another potential follow-up test is a Contraction Stress Test (CST), where mild uterine contractions are induced to observe the baby’s heart rate response under stress. An ultrasound may also be used independently to further evaluate the baby’s growth, anatomy, and blood flow. These subsequent tests provide a more complete picture of the baby’s health and help determine if any medical intervention is necessary.