A 32-year-old G1P0 presents to the antenatal testing unit at 32 weeks of gestation for a non-stress test (NST). Yesterday she had several episodes of diarrhea and vomited once after attending a company picnic 2 days ago. She took loperamide with symptomatic relief. She is worried that she is dehydrated but otherwise feels well and has recovered from her gastrointestinal illness. The patient was diagnosed with gestational diabetes after routine screening in the 2nd trimester and has been started on insulin after unsuccessful attempts at lifestyle management. She also has migraines and gastroesophageal reflux disorder. The patient has a 15-pack-year smoking history but quit smoking in her 1st month of pregnancy. She denies alcohol or drug use. Her temperature is 98.5°F (36.9°C), blood pressure is 122/82 mmHg, pulse is 84/min, and respirations are 14/min. A representative portion of the NST is shown in Figure A. Which of the following is the most likely explanation for this patient’s non-stress test (NST) results?
Maternal medication use
Normal results
Fetal sleep cycle
Maternal smoking
Fetal sleep cycle
This patient presents with a nonreactive NST on routine testing. The most common explanation for a nonreactive NST is the fetal sleep cycle.

Patients with gestational diabetes are at increased risk of fetal demise and other perinatal complications. They require closer monitoring during the 3rd trimester of pregnancy. The NST, which lasts 20 minutes, is the most common method of doing so. After 32 weeks, a reactive (i.e. “normal”) NST requires at least 2 episodes of 15-beat-per-minute accelerations lasting for at least 15 seconds. An NST with no accelerations is considered nonreactive. A common cause of a nonreactive NST is the fetal sleep cycle and it is reasonable to continue the test for an additional 20 minutes to increase the chance of monitoring the fetus while awake. Otherwise, a nonreactive NST should prompt vibroacoustic stimulation or further testing such as a biophysical profile (BPP).

Umana and Siccardi review the evidence regarding the use of the NST during pregnancy. They discuss how the test is a measurement of fetal cardiovascular function at the time of the test. They recommend monitoring the baseline fetal heart rate and variability as well as the presence of accelerations and decelerations when interpreting test results.

Figure/Illustration A shows a representative portion of a NST where the fetal heart rate remains within a narrow band without accelerations (red circle). This pattern is characteristically seen in a nonreactive NST that can be due to the fetal sleep cycle.

Incorrect Answers:
Answer B: Maternal dehydration generally should not affect the reactivity of an NST. Although severe dehydration may reduce amniotic fluid volume and therefore increase the baseline fetal pulse, it is unlikely to change the accelerations or variability in an NST. Though this patient had diarrhea the day before, her pulse is not elevated and any dehydration is probably not clinically significant.

Answer C: Maternal medication use can cause nonreactivity on an NST if drugs such as morphine or other sedatives are taken. Although this patient did take loperamide (an opioid), it is unlikely that this has a sedative effect on the fetus as it acts selectively on the mu-opioid receptors in the large intestine and does not cross the blood-brain barrier.

Answer D: Maternal smoking in close proximity to an NST has been linked to nonreactivity, but this patient has not smoked in several months. Mothers who smoke are counseled to avoid smoking on the day of their NST to avoid the effects of nicotine on their test results.

Answer E: A normal result is known as a reactive NST and would present with accelerations and no concerning or late decelerations. This is not a reactive NST due to the lack of appropriate accelerations.

Bullet Summary:
The most common cause of a non-reactive non-stress test is the fetal sleep cycle.