What tests are typically performed during the 3rd trimester of pregnancy?

Third Trimester Tests in Pregnancy

The standard third trimester tests include regular ultrasound examinations to monitor fetal growth, amniotic fluid assessment, non-stress tests, biophysical profiles, and group B streptococcus screening, with additional specialized testing based on risk factors. 1

Routine Third Trimester Ultrasound Examinations

Purpose and Components

• Fetal growth monitoring: Assesses estimated fetal weight, abdominal circumference, head circumference, and femur length to identify growth abnormalities 1
• Amniotic fluid assessment: Evaluates for oligohydramnios or polyhydramnios, which can indicate fetal complications 1
• Fetal position: Determines presentation (cephalic, breech, transverse) for delivery planning 1
• Placental location: Confirms placental position and rules out placenta previa 1

Timing and Frequency

• Standard ultrasound at 32-34 weeks for low-risk pregnancies 1
• More frequent monitoring (weekly or twice weekly) for high-risk pregnancies 2
• For monochorionic twin pregnancies, more intensive surveillance with ultrasounds every 2-3 weeks 1
• For dichorionic twin pregnancies without complications, ultrasounds every 3-4 weeks 1

Fetal Well-Being Assessment

Non-Stress Test (NST)

• Evaluates fetal heart rate response to movement
• Typically performed after 32 weeks in high-risk pregnancies
• A reactive NST (showing accelerations of fetal heart rate with movement) indicates good fetal oxygenation 2

Biophysical Profile (BPP)

• Comprehensive assessment with five components (each scored 0 or 2):
  • Breathing movements
  • Body/limb movements
  • Fetal tone
  • Amniotic fluid volume
  • NST (if included)
• Total score of 8-10/10 indicates well-compensated fetus
• Score of 6/10 requires repeat testing within 24 hours
• Score of ≤4/10 indicates potential fetal compromise requiring intervention 2

Doppler Ultrasound Studies

• Evaluates blood flow in umbilical artery, middle cerebral artery, and ductus venosus
• Particularly important for monitoring:
  • Intrauterine growth restriction
  • Twin-twin transfusion syndrome
  • Monochorionic twins
  • Maternal hypertensive disorders 1, 2
• Abnormal findings (absent or reversed end-diastolic flow) indicate increased risk of adverse outcomes and require more intensive monitoring 2

Maternal Laboratory Testing

Group B Streptococcus (GBS) Screening

• Universal screening at 36-37 weeks with vaginal-rectal swab
• Positive results require intrapartum antibiotic prophylaxis to prevent neonatal infection 1
• Physical examination alone without laboratory testing may be sufficient for monitoring infants born to GBS-colonized mothers who received adequate prophylaxis 3

Glucose Tolerance Testing (if not done earlier)

• For women not previously screened for gestational diabetes
• One-hour glucose challenge test or three-hour glucose tolerance test

Special Considerations for High-Risk Pregnancies

Preeclampsia Monitoring

• Blood pressure measurements
• Urine protein assessment
• Laboratory tests: Complete blood count, liver function tests, renal function tests
• Fetal biometry at first diagnosis of preeclampsia 2

Multiple Gestations

• More frequent ultrasound monitoring:
  • Monochorionic twins: Biometry every 2-3 weeks, weekly assessment for complications 1
  • Dichorionic twins: Every 3-4 weeks if uncomplicated 1
• Specialized monitoring for:
  • Twin-twin transfusion syndrome
  • Selective intrauterine growth restriction
  • Twin anemia-polycythemia sequence 1

Suspected Fetal Growth Restriction

• Increased frequency of ultrasound (every 2-3 weeks)
• Doppler studies of umbilical artery, middle cerebral artery, and ductus venosus
• More frequent NST or BPP (twice weekly) 2

Clinical Implications and Considerations

Detection Rates and Outcomes

• Third trimester ultrasound detects approximately 3.68 new fetal anomalies per 1000 women scanned 4
• Most commonly detected anomalies: urogenital (55%), central nervous system (18%), and cardiac (14%) 4
• Universal third trimester ultrasound in low-risk pregnancies may improve detection of growth abnormalities (number needed to diagnose: 7) 5
• However, routine third trimester ultrasound has not consistently shown improvement in overall perinatal outcomes in all studies 6

Management of Abnormal Findings

• For abnormal BPP (≤4): Consider immediate delivery if at term; if preterm, weigh risks of prematurity against risks of continued pregnancy 2
• For growth restriction: Increase monitoring frequency and consider timing of delivery based on severity and gestational age 1, 2
• For oligohydramnios: More frequent monitoring and possible intervention depending on gestational age and severity 1

Third trimester testing should be tailored to the specific risk factors present, with more intensive monitoring for high-risk conditions to optimize maternal and fetal outcomes.