Pregnancy involves various medical checks to ensure the baby’s health and development. Among these is the Non-Stress Test (NST), a common prenatal evaluation that assesses the baby’s well-being inside the womb.
Understanding the Non-Stress Test
A Non-Stress Test (NST) monitors a baby’s heart rate in response to their own movements. It is called “non-stress” because it observes natural activity without interventions. During the test, two elastic belts are placed around the pregnant person’s abdomen: one tracks uterine contractions, and the other monitors the baby’s heart rate. The baby’s heart rate should increase when they move or kick, similar to how an adult’s heart rate rises with physical activity.
The NST assesses if adequate oxygen supply supports the baby’s activity and heart rate. It specifically looks for accelerations, which are temporary increases in the baby’s heart rate. These accelerations, particularly when occurring with movement, indicate good fetal health and responsiveness. The test typically lasts 20 to 40 minutes.
Standard Timing of NSTs
Non-Stress Tests typically begin in the later stages of pregnancy, usually after 28 weeks of gestation. This timing is when a baby’s heart rate consistently reacts to movements, aligning with the third trimester (weeks 29 to 40).
For many low-risk pregnancies, an NST may not be necessary. However, if monitoring is indicated, some healthcare providers may begin NSTs around 32 to 34 weeks. The exact timing is often individualized, depending on specific circumstances and the healthcare provider’s assessment.
Medical Reasons for an NST
Healthcare providers recommend a Non-Stress Test when specific maternal or fetal conditions indicate a need for closer monitoring of the baby’s well-being. These tests are often part of the management for high-risk pregnancies. Conditions such as maternal diabetes, especially type 1, or high blood pressure, including preeclampsia, frequently warrant an NST to assess fetal oxygen supply.
A decrease in the baby’s usual movement patterns is another common reason for an NST. If pregnancy extends beyond 40 weeks, an NST may be performed. Carrying multiple babies, such as twins or triplets, often leads to regular NSTs. Concerns about the baby’s growth (small for gestational age), or abnormal amniotic fluid levels, can also prompt this monitoring. Additionally, a history of previous pregnancy complications, such as a stillbirth, may lead to earlier or more frequent Non-Stress Tests, sometimes starting at 28 weeks.
Interpreting NST Outcomes
The results of a Non-Stress Test are typically categorized into two primary outcomes: “reactive” or “non-reactive.” A “reactive” NST signifies a reassuring result, indicating healthy fetal heart rate patterns. This means the baby’s heart rate has increased at least twice by a certain amount (e.g., 15 beats per minute above baseline for 15 seconds) within a 20-minute period when the baby moves.
Conversely, a “non-reactive” NST indicates that further evaluation is necessary, but it does not automatically mean there is a problem. A non-reactive result might occur if the baby is asleep during the test, or it could be influenced by certain medications the pregnant person is taking. In such cases, the healthcare provider may attempt to wake the baby using a gentle buzzing device or by having the pregnant person consume a sugary drink. If the NST remains non-reactive, common follow-up tests may be recommended, such as a biophysical profile (BPP), which includes an ultrasound, or a contraction stress test, to gather more information about the baby’s well-being.