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Introduction ⬇

An NST is a prenatal screening test to assess fetal well-being. It can be performed in a hospital, a clinic, practitioners’ office, or possibly a home care setting. The NST is usually performed in the third trimester. It can be safely done once a week and may be performed twice a week with high-risk pregnancies. Test results reflect the functions of the fetal brain stem, autonomic nervous system, and heart.

Procedure ⬆ ⬇

  1. Assess maternal vital signs, last oral intake (including medicines or street drugs), smoking history, and fetal movement history.

  2. Apply external fetal monitor with the woman positioned off her back in lateral tilt position.

  3. After 26 weeks of gestation, this assessment of the FHR pattern without contractions evaluates fetal oxygenation. Fetal movement may or may not be identified by the woman during the test. If gestation is <26–30 weeks, the fetus may not meet the criteria for a reactive NST yet may be a healthy fetus.

  4. If unable to elicit FHR accelerations during NST, acoustic stimulation of a fetus that is not acidotic may evoke FHR accelerations that seem to predict fetal well-being. An artificial larynx (vibroacoustic stimulator) that is designed for fetal monitoring is placed on the patient’s abdomen, and the stimulus is activated for 1–2 seconds. The stimulus may be repeated up to three times for gradually increased durations up to 3 seconds to bring about FHR accelerations. Use of acoustic stimulation can shorten the time needed for reactive NST and reduce false-positive test results.

Procedural Alert

  1. A nonreactive NST (positive test) should be followed by a CST.

  2. Ultrasound studies and a fetal biophysical profile (FBP) may be needed after a nonreactive NST.

Clinical Implications ⬆ ⬇

A nonreactive NST (positive test) consists of fewer than two accelerations of FHR (ACOG criteria). If the fetus does not react within the first 20 minutes, stimulation should be applied. The test is considered nonreactive if, after extension to 40 minutes, the ACOG criteria are not met. This extended testing minimizes the possibility of lack of activity owing to fetal sleep. If the FHR pattern is unclear, the test is considered inconclusive or unsatisfactory.

Clinical Alert

  1. Nonrepetitive, brief (<30 seconds) variable decelerations may be noted in up to 50% of NSTs and do not indicate a compromised fetus or a need for interventions.

  2. Repetitive variable decelerations (three in 20 minutes) or decelerations that last 60 seconds or longer indicate nonreassuring FHR pattern and increased risk for cesarean delivery.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Advise the patient regarding the need for weekly or twice-weekly testing according to the healthcare provider’s orders if pregnancy history indicates risk factors for antepartum fetal demise. If NST is performed for a single occurrence of decreased fetal movement in uncomplicated pregnancy and reactive NST results, reassure the patient that the test need not be repeated.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

A false-positive result may be caused by fetal sleep, preterm gestation, smoking before the NST, congenital anomalies, or maternal use of drugs such as central nervous system depressants or beta-blockers.

Reference Values ⬆

Normal