The Stress of a Nonstress Test

Q: I am supposed to have a nonstress test. Could you explain what this is?

A: A nonstress test (NST) is a test done during the last three to four months of pregnancy in certain high-risk situations, usually to evaluate the fetus' health.

The NST is an "external" test, which means no invasive procedures are necessary to perform the test. It can be done in the doctor's office, a hospital or a fetal-evaluation unit. An NST normally takes 20 to 60 minutes.

The NST is called a "nonstress" test because it evaluates the fetus without causing it any stress. A stress test, also known as a "contraction stress test," involves hospital admission and induction of contractions to determine fetal health.

Reasons for doing a nonstress test include these high-risk conditions:

• Decreased fetal movements
• Going beyond the due date
• Diabetes
• High blood pressure
• Changes in fetal growth rate

The NST is usually done while you lie down or sit in a comfortable chair. Two small monitors are placed on your abdomen and held in place using elastic belts. One monitor checks for contractions; the other records the fetal heartbeat. The two monitors are attached by cables to a larger machine, the electronic fetal monitor. You're given a pushbutton and asked to push it every time you feel the fetus move. Your button pushes are recorded and then compared to the fetal heart-rate tracing, the number of fetal movements, and the response of the fetus' heart to the movements.

On this nonstress test reading, each small horizontal box represents a 10-second interval. The upper line shows the fetal heart rate at an average of 140 beats per minute; the lower line shows uterine activity. Each time the fetus moves, the fetal heart rate accelerates from about 140 to 160 or 170 beats per minute, stays up for 20 to 30 seconds, then drops back to 140 beats per minute. This is a reassuring result.

The test results may be either reactive or non-reactive.

• A reactive NST is one that shows two or more fetal movements accompanied by a fetal heart-rate acceleration of at least 15 beats per minute lasting at least 15 seconds during a 15- to 20-minute period. A reactive NST usually means that the fetus is healthy and is getting enough nutrition. Lack of fetal movement for short periods of time (20 to 40 minutes) is usually of no concern and may simply occur because the fetus is asleep.

• A nonreactive NST shows fetal movements without an increase in fetal heart rate. A nonreactive NST doesn't necessarily mean there's a problem. In fact, in a large proportion of cases, the fetus is healthy despite the nonreactive NST. A nonreactive result usually indicates that further testing should be done, such as a biophysical profile or a contraction stress test. The absence of fetal movement for more than an hour is cause for concern. In that case the doctor may order a repeat NST, after the mother has a meal, or advise early delivery.

Page 1 The Stress of a Nonstress Test
Page 2 Checking on the Fetus in a High-Risk Pregnancy

 

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