A contraction stress test (CST), also known as an oxytocin challenge test (OCT) is performed to assess fetal heart rate (FHR) in response to uterine contractions (stimulated by oxytocin) through electronic fetal monitoring (EFM). Types of CST include oxytocin administration and nipple stimulation CST, also known as a breast stimulation test. Nipple stimulation promotes the release of oxytocin. These tests are performed after 28 weeks of gestation, when a nonstress test (NST) is nonreactive. Continuous EFM is used. Because uterine contractions are associated with a reduction in uteroplacental blood flow, spontaneous, oxytocin-induced, or nipple stimulation–induced contractions with a frequency of three in 10 minutes may be used clinically as a standard test of fetoplacental respiratory function. Stress of this magnitude has been proved clinically useful in separating fetuses with suboptimal oxygen reserve from those with adequate reserve (the vast majority), and it does not significantly compromise the normal fetus.

1. Position the patient on the left side after strapping an external fetal monitor and a device for monitoring contractions (tocodynamometer) to the patient’s abdomen.

2. Administer oxytocin or have the patient perform nipple stimulation, as ordered.

3. Monitor uterine activity and fetal heart response.

1. The presence of consistent and persistent late decelerations with most uterine contractions, regardless of their frequency, constitutes a positive (abnormal) OCT result. This is often associated with decreased baseline FHR variability, a lack of FHR acceleration with fetal movement, and a fetus at risk for intrauterine asphyxia.

2. The results of OCT can be categorized as follows:

   1. Negative: No late decelerations

   2. Positive: Late decelerations following 50% or more of contractions, even if the frequency of the contractions is fewer than three in 10 minutes

   3. Equivocal: Intermittent, late, or variable decelerations

   4. Unsatisfactory: Fewer than three contractions within 10 minutes or a poor-quality tracing

Pretest Patient Care

1. Explain the reason for testing. Tell the patient that the test may take 2 hours or more.

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

Normal

• Negative: No late decelerations associated with at least three contractions within a 10-minute period.

• A normal (negative) CST implies that placental support is adequate, that the fetus is probably able to tolerate the stress of labor should it begin within 1 week, and that there is a low risk for intrauterine death due to hypoxia.