What is the significance of tests ordered for a patient in prelabor?

Significance of Tests Ordered for Patients in Prelabor

Tests ordered for patients in prelabor are critical for identifying fetuses at risk of intrauterine death or injury from antepartum asphyxia, particularly in high-risk pregnancies, though they have limited value in low-risk populations. 1

Primary Purpose of Antepartum Testing

The fundamental goal of prelabor testing is reducing the risk of stillbirth by detecting signs of fetal compromise that can guide the timing of delivery to optimize outcomes. 1, 2

• High-risk pregnancies benefit most from serial surveillance, as these patients face 2-fold to 40-fold increased risk of stillbirth compared to the baseline 5 per 1,000 in low-risk pregnancies. 1
• All antepartum tests share high negative predictive values (>99.9%), meaning a normal result is highly reassuring that stillbirth is unlikely within the next week. 1
• No single test has proven superior to others, but all serve the same critical function of identifying fetuses requiring intervention. 1

Key Testing Modalities and Their Significance

Ultrasound-Based Assessments

Biophysical Profile (BPP) evaluates four parameters (fetal breathing, body movements, tone, and amniotic fluid volume) with or without nonstress testing. 1

• The likelihood of stillbirth within 1 week of a normal BPP is only 0.8 per 1,000 cases. 1
• BPP is more specific than sensitive, performing well at identifying healthy fetuses but poorly at recognizing those at risk. 1

Modified Biophysical Profile (mBPP) combines nonstress testing with amniotic fluid assessment. 1

• Performs comparably to full BPP with similar negative predictive value (<1 per 1,000 stillbirths within one week). 1
• Allows assessment of both acute (nonstress test) and chronic (amniotic fluid) markers of fetal status. 1

Doppler Velocimetry is particularly significant for pregnancies with suspected intrauterine growth restriction. 1

• Absent or reversed end-diastolic flow in the umbilical artery is associated with >20% perinatal mortality and is the most predictive finding for adverse outcomes. 1
• Abnormal arterial waveforms typically precede venous abnormalities and biophysical changes in the characteristic sequence of fetal compromise. 1

Timing and Frequency

Weekly or twice-weekly testing has become standard practice in high-risk pregnancies, though this frequency is not based on rigorous scientific evidence. 1

• Testing typically initiates at 32-34 weeks' gestation but should be individualized based on the specific risk factor, gestational age, and likelihood of neonatal survival. 1
• For conditions like intrahepatic cholestasis of pregnancy, testing should begin at a gestational age when delivery would be performed in response to abnormal results. 1

Critical Limitations and Pitfalls

Antepartum testing cannot predict acute events such as placental abruption or cord accidents, which account for a significant proportion of stillbirths. 1

• Up to half of all stillbirths occur in patients without recognized risk factors, limiting the overall impact of selective surveillance. 1
• In intrahepatic cholestasis of pregnancy specifically, stillbirths have occurred within days of reactive nonstress tests, suggesting testing may not be effective for conditions causing sudden fetal death. 1

Low-risk pregnancies should NOT undergo routine testing, as there is no evidence of benefit and false-positive results may lead to iatrogenic prematurity from unnecessary interventions. 1

• Routine umbilical artery Doppler in unselected populations shows no maternal or perinatal benefits. 1
• False-positive results can increase cesarean delivery rates and complications from premature delivery. 1

High-Risk Conditions Warranting Testing

Testing is indicated for specific maternal, fetal, and obstetric risk factors: 1

Maternal conditions: Advanced age, obesity, hypertensive disorders, cyanotic heart disease, thrombophilia, diabetes, thyroid disorders, chronic renal disease, connective tissue disease, cholestasis, hemoglobinopathies, isoimmunization, history of unexplained stillbirth. 1

Fetal conditions: Intrauterine growth restriction (the strongest evidence for testing benefit), structural anomalies, genetic syndromes, fetal arrhythmias, blood group incompatibilities, fetal anemia, congenital infections, multiple gestations. 1

Obstetric complications: Decreased fetal movement, preterm premature rupture of membranes, postdates pregnancy, abnormal maternal serum markers, placental abruption, vaginal bleeding, amniotic fluid abnormalities. 1

Management of Abnormal Results in Preterm Pregnancies

Before term, risks of prematurity must be weighed against risks of intrauterine death when test results are abnormal. 1

• Repeat testing or alternative tests are usually indicated rather than immediate delivery to avoid iatrogenic prematurity. 1
• Serial surveillance continues weekly or twice-weekly depending on the indication and ongoing results until delivery. 1