If your pregnancy is considered high-risk, your doctor may want to start administering fetal non-stress tests sometime after the 30th week of pregnancy. A fetal non-stress test (called an NST)—so named because it is non-invasive and puts no stress on the developing fetus—is designed to determine fetal “well-being” by detecting changes in the fetal heartbeat, and it is administered if the doctor is concerned that the fetus may not be getting enough oxygen. In some cases it is done just once for a particular reason, and sometimes it is done on a regular basis until delivery.
Reasons to Perform a Fetal Non-Stress Test
This test may be conducted for a number of reasons. If you have indicated to your doctor that your baby does not seem to be moving around as much as before, or if other testing indicates that there is reason to suspect abnormalities affecting the umbilical cord or the placenta, your doctor may want to conduct a non-stress test.
Other reasons for this test include:
- You reported a recent fall or car accident (or something along those lines)
- You are past your due date
- You have high blood pressure or diabetes
- You have a thyroid condition
- Your baby is not growing as rapidly as expected (possibly due to intrauterine growth restriction)
- You have third-trimester bleeding
- You are 40 years old or over
- You are carrying twins or multiple fetuses
- Previous pregnancies have ended in miscarriage, stillbirth, or other pregnancy loss
- You have too much or too little amniotic fluid in your uterus
- There is reason to believe you have become Rh sensitized, and your baby has Rh-positive blood; this can cause a rare condition known as Rh disease
Depending on the reason for the test, your doctor may recommend additional non-stress testing throughout the course of your pregnancy, possibly even once or twice a week until delivery.
What Should I Expect During the Procedure, and How Should I Prepare?
A fetal non-stress test (NST) is typically conducted in a doctor’s office. You may be asked to eat something or maybe drink some juice before the test, since some babies are more active after their mothers have eaten. After you arrive at the office and check in, you will be advised to empty your bladder for your own comfort. You will then be shown to an examination room and asked to lie down, and your blood pressure will be taken prior to the test.
The device used is similar to or sometimes is exactly the same type of fetal monitor that is used during labor. To begin the test, two stretchable belts will be placed around your abdomen. These belts will have monitors attached to them to record the baby’s heart rate, and to monitor uterine contractions if you have them.
You may be asked to indicate the baby’s movements to the doctor or nurse administering the test by pressing a button whenever the baby moves. The doctor or nurse will be looking for an increase in the baby’s heart rate, which should coincide with each movement. However, recent modifications of the NST protocol no longer depend on matching fetal heart rate changes to fetal movement, so some practices no longer have the patient mark the movement.
A fetal non-stress test typically lasts about twenty minutes, although it may take as long as forty minutes if the baby is asleep or not especially active. If the baby appears to be sleeping, the doctor may attempt to awaken him (or her!) by using a small noise-making or vibrating device, or by simply having you drink a glass of juice or cold water.
How Are the Results of a Fetal Non-Stress Test Interpreted?
The results are classified as either reactive or non-reactive. The heart rate of a healthy, reactive fetus should increase when it moves, and should increase periodically regardless of fetal movement). This should happen at least twice during the twenty- or thirty-minute testing period. Do not be concerned if you do not feel the baby’s movements while the test is being administered; the device used to monitor the baby’s heart rate will pick up the increase in heart rate even if the baby’s movement is too subtle for you to be aware of it. A reactive NST is also sometimes called a reassuring fetal heart rate test.
At the beginning of the test, a baseline fetal heart rate will be determined. A fetus that is older than 32 weeks will be considered reactive if at least twice during a 20-minute window its heart rate exceeds that baseline by a minimum of 15 BPM (beats per minute) for at least 15 seconds. A younger fetus will be considered reactive if the heart rate increases by a minimum of 10 BPM for at least 10 seconds.
What Happens if My Baby is Considered “Non-Reactive”?
If your baby’s responses to the fetal non-stress test do not meet the criteria described above, further testing will be ordered. There are a number of reasons why this may have happened—for example, a “non-reactive” fetus may simply have slept through the test despite the doctor’s attempts to wake him or her.
If this happens, your doctor will most likely order another non-stress test, and possibly also arrange for a biophysical profile. The biophysical profile combines a non-stress test with fetal ultrasound testing in order to evaluate the baby’s movement and the level of amniotic fluid in your uterus.
Contraction stress tests can also be used to look for changes in the fetal heart rate that coincide with uterine contractions (contractions can be induced using intravenous Pitocin, or sometimes nipple stimulation), but this type of test has fallen out of favor and is rarely done.
Why Are These Tests Done?
The purpose of these tests is to determine fetal “well-being.” This non-specific term is meant to assure the medical care providers that the baby is getting enough blood flow, oxygen, and nutrients from the placenta, and therefore interventions such as delivery of the baby or admission to the hospital for more intensive monitoring are highly unlikely to be needed.
A reactive NST provides “reassurance” that the baby is doing well. Depending on the reason for the test, it might be repeated again in a few days or perhaps a week; in other cases it may only be a one-time test due to an episode of decreased fetal movement that has resolved.