Antepartum Testing

Introduction

The goals of antepartum testing are (1) to identify fetuses in jeopardy so that permanent injury or death might be prevented and (2) to identify healthy fetuses so that unnecessary intervention can be avoided. The key measure of an antepartum test is the false-negative rate, usually defined as the incidence of fetal death within 1 week of a normal antepartum test. Another important measure is the false-positive rate. A false-positive test usually is defined as an abnormal test that prompts delivery but is not associated with evidence of acute disruption of fetal oxygenation (meconium, intrapartum “fetal distress” or low Apgar scores) or chronic disruption of fetal oxygenation (fetal growth restriction, oligohydramnios).

Antepartum testing is used primarily in patients at increased risk for disrupted fetal oxygenation. Common obstetric and medical indications for antepartum testing are summarized in Box 46.1. For most medical indications, testing is initiated by 32–34 weeks. Tables 46.1 and 46.2 summarize the usual timing of antepartum testing for medical and obstetric indications.

Table 46.1 Timing of initiation of antepartum testing for medical indications

Medical indications	Timing
Gestational diabetes (diet controlled)	40 weeks
Gestational diabetes (insulin)	32–34 weeks
Type 1 or 2 diabetes	32–34 weeks
Chronic hypertension	32–34 weeks
Cyanotic cardiac disease	32–34 weeks
Renal disease	32–34 weeks
Thyroid disease	32–34 weeks
Collagen vascular disease	32–34 weeks
Pulmonary disease (severe asthma)	32–34 weeks
Hemoglobinopathy	32–34 weeks

Table 46.2 Timing of initiation of antepartum testing for obstetric indications

Obstetric indications	Timing
Post-term pregnancy	40–41 weeks
Unexplained elevated AFP, hCG	32–34 weeks
Cholestasis of pregnancy	32–34 weeks
Antiphospholipid antibody syndrome	32–34 weeks
Previous unexplained stillbirth	32–34 weeks*
Suspected fetal growth restriction	At diagnosis
Decreased fetal movement	At diagnosis
Pre-eclampsia	At diagnosis
Multiple gestation (discordant)	At diagnosis
Alloimmunization	At diagnosis
Oligohydramnios	At diagnosis
* Or 1 week earlier than previous loss.

The contraction stress test (oxytocin challenge test)

The contraction stress test (CST) or oxytocin challenge test (OCT) arose from intrapartum observations linking late decelerations with poor perinatal outcome. The test seeks to identify transient fetal hypoxemia by demonstrating late decelerations in fetuses exposed to the stress of spontaneous (CST) or induced (OCT) uterine contractions. Kubli and associates found that late decelerations occurring during spontaneous uterine contractions were associated with increased rates of fetal death, growth retardation and neonatal depression [1]. Similar observations were made by other investigators using oxytocin or nipple stimulation to provoke uterine contractions. The CST is performed weekly and is interpreted as summarized in Box 46.2.

Box 46.1 Indications for antepartum testing

Obstetric indications

Post-term pregnancy

Unexplained elevated AFP, hCG

Cholestasis of pregnancy

Antiphospholipid syndrome

Previous unexplained stillbirth

Suspected fetal growth restriction

Decreased fetal movement

Pre-eclampsia

Multiple gestation (discordant)

Alloimmunization

Oligohydramnios

Medical indications

Diabetes

Chronic hypertension

Cyanotic cardiac disease

Renal disease

Thyroid disease

Collagen vascular disease

Pulmonary disease (severe asthma)

Hemoglobinopathy

The CST is considered negative if there are at least three uterine contractions in a 10-minute period with no late decelerations on the tracing. In this case, the routine testing schedule is resumed. Unsatisfactory, suspicious or equivocal tests require repeat testing the following day. Usually, a positive CST or OCT warrants hospitalization for further evaluation and/or delivery. Freeman and colleagues tested more than 4600 women with the CST and reported a false-negative rate of 0.4/1000 [2]. Reported false-positive rates range from 8% to 57% with an average of approximately 30% [3]. The advantages of this form of testing include excellent sensitivity and a weekly testing interval. Limitations include a high rate of equivocal results requiring repeat testing, increased expense and inconvenience (particularly if oxytocin is required), and increased time requirement compared to nonprovocative tests. Additionally, the CST is contraindicated in several clinical settings, including preterm labor, placenta previa, vasa previa, cervical incompetence, multiple gestation and previous classic cesarean.

The nonstress test

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