The Fetal Nonstress Test (NST) is a method to evaluate the viability of a fetus. It documents the placenta’s ability to provide an adequate blood supply to the fetus. The NST can be used to evaluate any high-risk pregnancy in which fetal well-being may be threatened. These pregnancies include those marked by diabetes, hypertensive disease of pregnancy (toxemia), intrauterine growth restriction, Rh-factor sensitization, history of stillbirth, postmaturity, or low estriol levels.
The NST is a noninvasive study that monitors acceleration of the fetal heart rate (FHR) in response to fetal movement. This FHR acceleration reflects the integrity of the central nervous system (CNS) and fetal well-being. Fetal activity may be spontaneous, induced by uterine contraction, or induced by external manipulation. Oxytocin stimulation is not used. Fetal response is characterized as “reactive” or “nonreactive.” The NST indicates a reactive fetus when, with fetal movement, two or more FHR accelerations are detected, each of which must be at least 15 beats/min for 15 seconds or more within any 10-minute period.
The test is 99% reliable in indicating fetal viability and negates the need for the fetal contraction stress test. If the test detects a nonreactive fetus (i.e., no FHR acceleration with fetal movement) within 40 minutes, the patient is a candidate for the CST. A 40-minute test period is used because this is the average duration of the sleep-wake cycle of the fetus. The cycle may vary considerably, however.
The NST is useful in screening high-risk pregnancies and in selecting those patients who may require the CST. An NST is now routinely performed before the CST to avoid the complications associated with oxytocin administration. No complications are associated with the NST.
Indication of Nonreactive Fetus to the Nonstress Test
This result alone does not indicate fetal distress, but when it is combined with other noninvasive tests such as CST, biophysical profile, alpha-fetoprotein, pregnanediol, and obstetric ultrasound, fetal health can be accurately determined.