44 Obstetrics and Gynecology Board Review •••
❍ What are the contraindications to the CST?
Patients at high risk of premature labor such as patients with premature rupture of membranes, multiple gestation,
and cervical incompetence and those patients in which uterine contractions should be avoided such as placenta
previa, previous classical cesarean section, or previous uterine surgery.
❍ How is a CST interpreted?
According to the presence or absence of late FHR decelerations.
- Negative CST: No late or significant variable decelerations.
- Positive CST: Late decelerations following 50% or more of contractions.
- Equivocal-suspicious: Intermittent late decelerations or significant variable decelerations.
- Equivocal-hyperstimulatory: FHR decelerations that occur when contraction pattern is more frequent than every
2 minutes or lasting longer than 90 seconds. - Unsatisfactory: Fewer than three contractions in 3 minutes or an uninterpretable tracing.
❍ What is the incidence of perinatal death within 1 week of a negative CST?
0.4/1000.
❍ What is the likelihood of perinatal death after a positive CST?
7 to 15%.
❍ Is a positive CST an indication for an elective cesarean section?
No. A trial of labor can be attempted if the cervix is favorable for induction so that FHR monitoring and uterine
contractility monitoring can be carefully assessed.
❍ When should a suspicious or equivocal CST be repeated?
Within 24 hours.
❍ How is a NST performed?
The patient is seated in a reclining chair and tilted to the left slightly with a Doppler ultrasound transducer
monitoring the FHR and a tocodynamometer detecting uterine contractions.
❍ What defines a reactive NST?
Presence of at least two accelerations of the FHR in 20 minutes of monitoring [15 beats per minute (bpm) by
15 seconds in ≥32 weeks and 10 bpm by 10 seconds in <32 weeks].
❍ What pathway is required for a healthy fetus to exhibit accelerations above the baseline FHR?
An intact neurologic coupling between the CNS and the fetal heart.
❍ What fetal condition can disrupt this pathway?
Fetal hypoxia, metabolic acidosis, or CNS anomalies.
❍ What is the most common cause of absent FHR accelerations?
Fetal sleep state.