The Non-Stress Test (NST) is a common, non-invasive method used during pregnancy to assess the health of the developing fetus. This assessment tool monitors the baby’s heart rate response to movement. The procedure is named “non-stress” because it places no physical strain on the mother or the baby. The NST offers valuable insight into the baby’s environment inside the uterus.
The Purpose of the Non-Stress Test
The goal of the Non-Stress Test is to confirm that the fetus is receiving sufficient oxygen and has a properly functioning nervous system. The test relies on the principle that a healthy, well-oxygenated fetus will spontaneously increase its heart rate in response to physical activity. This temporary rise is known as “fetal reactivity.”
The test observes for accelerations, which are temporary increases in the fetal heart rate above its baseline. These accelerations demonstrate the coupling between the fetal central nervous system and the heart is intact. A “reactive” result is a strong indicator of fetal well-being and adequate oxygenation. Conversely, a lack of reactivity can suggest the baby may be asleep or experiencing insufficient oxygen.
Standard Timing for NST Initiation
For low-risk and uncomplicated pregnancies, the Non-Stress Test is not typically initiated until the late stages of the third trimester. Standard timing usually begins around 32 to 34 weeks of gestation. This timing is based on a physiological milestone in fetal development.
Before this gestational age, the baby’s nervous system is often not mature enough to reliably produce the required heart rate accelerations consistently. Testing earlier would frequently result in a “non-reactive” result, even in a healthy baby, leading to unnecessary concern and additional testing. For low-risk pregnancies, the NST is often only used if the baby remains in utero past the due date to monitor for post-term complications.
Conditions Requiring Earlier Testing
While 32 to 34 weeks is the standard for low-risk pregnancies, several high-risk conditions necessitate beginning NSTs earlier. Monitoring may start as early as 28 to 32 weeks, depending on the severity and nature of the risk. The timing is adjusted when there is concern that the uterine environment or placental function may be compromised.
Maternal medical conditions such as poorly controlled diabetes or chronic hypertension often require earlier, frequent monitoring. Preeclampsia and any condition leading to restricted fetal growth also trigger earlier testing. The NST helps track if these maternal issues are affecting the baby’s oxygen supply and overall health.
Testing is also initiated if the mother reports a notable decrease in fetal movement, as this can be a symptom of fetal compromise. Other indications include oligohydramnios (low amniotic fluid volume). Monitoring is also advanced for complications in multiple gestations or concerns about Rh incompatibility.
What Happens During the Test
The Non-Stress Test is typically conducted in an outpatient clinic or hospital setting. The patient lies comfortably in a semi-reclined position while two elastic belts, each containing a sensor, are placed around the abdomen. One sensor uses ultrasound to monitor the fetal heart rate, and the second records any uterine contractions.
The test generally lasts a minimum of 20 minutes, extending up to 40 minutes if the baby is inactive or sleeping. During the test, the mother may be asked to press a button each time she feels movement, allowing the provider to correlate movement with the heart rate tracing. The results are usually available immediately, with the goal being to achieve a “reactive” tracing.