Third Trimester Tests and Screening
A Comprehensive Guide for Nursing Students
Table of Contents
Introduction to Third Trimester Screening
The third trimester (weeks 28-40) is a critical period for assessing fetal well-being and preparing for birth. Various screening tests are performed during this period to identify potential complications and ensure optimal outcomes for both mother and baby.
As nursing professionals, understanding these tests is essential for providing quality patient education, recognizing abnormal findings, and implementing appropriate interventions.
Memory Aid: T-SCAN
Remember the main purposes of third trimester screening with T-SCAN:
- Track fetal growth and development
- Screen for gestational diabetes
- Check for Group B Strep
- Assess placental function
- Note fetal well-being and distress
Group B Streptococcus Screening
Overview
Group B Streptococcus (GBS) is a type of bacteria that can colonize the vaginal and rectal areas. While typically harmless to the mother, it can cause serious infections in newborns if transmitted during delivery.
Procedure
- Performed between 36-38 weeks gestation
- Involves swabbing the lower vagina and rectum
- Sample is cultured for GBS bacteria
- Results typically available in 24-48 hours
- Test is simple, painless, and doesn’t harm the baby
Key Points
- About 25% of pregnant women carry GBS
- GBS can cause sepsis, pneumonia, or meningitis in newborns
- CDC recommends universal screening
- Positive result requires intrapartum antibiotic prophylaxis
Nursing Implications
- Educate patients that GBS colonization is common and not an STI
- Explain that antibiotics during labor can prevent transmission
- Document GBS status prominently in patient records
- Prepare for intravenous antibiotic administration during labor if positive
Memory Aid: GBS-TAP
Remember key aspects of GBS management with GBS-TAP:
- GBS screening at 36-38 weeks
- Bacteria identification via culture
- Swab both vaginal and rectal areas
- Treatment with antibiotics during labor
- Assess for risk factors if status unknown
- Prevent neonatal infection as primary goal
Glucose Challenge & Glucose Tolerance Tests
Gestational diabetes screening is crucial for identifying pregnant women who develop glucose intolerance during pregnancy. While typically performed in the second trimester (24-28 weeks), it may be conducted in the third trimester for women with risk factors or those who missed earlier screening.
| Test Type | Procedure | Interpretation |
|---|---|---|
| One-Hour Glucose Challenge Test (GCT) |
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| Three-Hour Glucose Tolerance Test (GTT) |
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Nursing Considerations
- Some patients may experience nausea after consuming glucose solution
- For GTT, verify patient has fasted appropriately
- Ensure timely blood draws at specified intervals
- Document any symptoms experienced during testing
- Prepare to educate patients diagnosed with gestational diabetes about monitoring and management
Memory Aid: SWEET
Remember glucose testing protocol with SWEET:
- Screening with 50g glucose challenge
- Waiting one hour for blood draw
- Elevated value (≥140 mg/dL) requires further testing
- Extended testing with 3-hour GTT if necessary
- Two or more abnormal values confirm gestational diabetes
Biophysical Profile (BPP)
Overview
The Biophysical Profile (BPP) is a comprehensive assessment of fetal well-being that combines ultrasound evaluation with a non-stress test. It provides a more complete picture of fetal status than any single test alone.
Components Assessed (5 Parameters)
- Fetal breathing movements: At least one episode of ≥30 seconds within 30 minutes
- Gross body movements: At least 3 discrete movements within 30 minutes
- Fetal tone: At least one episode of extension with return to flexion
- Amniotic fluid volume: At least one pocket measuring ≥2 cm in two perpendicular planes
- Non-stress test (NST): Reactive heart rate pattern (may be omitted in modified BPP)
Indications
- Post-term pregnancy (≥41 weeks)
- Decreased fetal movement
- Intrauterine growth restriction (IUGR)
- Maternal conditions (hypertension, diabetes)
- Previous stillbirth
- Oligohydramnios or polyhydramnios
| Score | Interpretation | Management |
|---|---|---|
| 8-10/10 (with normal AFV) | Reassuring/Normal | Routine follow-up appropriate for gestational age |
| 6/10 | Equivocal | Repeat testing within 12-24 hours or consider delivery if ≥37 weeks |
| 4/10 or less | Abnormal | Consider immediate delivery if fetus is viable and gestational age appropriate |
| Any score with abnormal AFV | Concerning | Further evaluation needed; may require delivery |
Nursing Implications
- Explain the procedure and purpose to reduce maternal anxiety
- Position patient comfortably for the examination (semi-Fowler’s or left lateral)
- Document BPP components and total score
- Report abnormal findings promptly
- Prepare for possible delivery if score indicates fetal compromise
Memory Aid: BBTAM
Remember BPP components with BBTAM:
- Breathing movements
- Body movements
- Tone of muscles
- Amniotic fluid volume
- Monitoring of heart rate (NST)
Non-Stress Test (NST)
Overview
The Non-Stress Test (NST) is a simple, non-invasive test that evaluates fetal heart rate patterns in response to fetal movement. It’s based on the principle that a healthy fetus will demonstrate temporary acceleration of heart rate with movement, indicating intact autonomic nervous system function.
Procedure
- External monitors placed on maternal abdomen
- Tocodynamometer measures uterine activity
- Doppler ultrasound transducer records fetal heart rate
- Maternal perception of fetal movement documented by pressing event marker
- Monitoring typically continues for 20-30 minutes
- May be extended to 40-60 minutes if initial period inconclusive
- No maternal/fetal stimulation applied (hence “non-stress”)
Indications
- Decreased fetal movement
- High-risk pregnancies
- Maternal conditions (hypertension, diabetes)
- History of previous stillbirth
- Post-term pregnancy
- Intrauterine growth restriction
- Multiple gestation
- Routine monitoring in late third trimester
| Result | Criteria | Interpretation/Action |
|---|---|---|
| Reactive |
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| Non-reactive |
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Nursing Considerations
- Position patient in semi-Fowler’s or left lateral position to prevent supine hypotension
- Recommend patient eat before testing (increases fetal activity)
- Document maternal vital signs before and during test
- Accurately record perceived fetal movements
- Recognize that fetal sleep cycles (20-40 minutes) may affect results
- Ensure proper placement of monitors for accurate readings
Memory Aid: REACTIVE
Remember NST reactive criteria with REACTIVE:
- Reassuring when criteria met
- Evaluate for at least 20 minutes
- Accelerations ≥15 beats per minute
- Count at least 2 accelerations
- Time duration ≥15 seconds each
- Indicates intact autonomic system
- Verify baseline variability is normal
- Ensure proper monitoring technique
Contraction Stress Test (CST)
Overview
The Contraction Stress Test (CST) evaluates how the fetus responds to the stress of uterine contractions, which temporarily decrease uteroplacental blood flow. It helps identify fetuses that might not tolerate the stress of labor. This test is used less frequently than NST or BPP due to its contraindications and complexity.
Procedure
- External monitors placed as in NST
- Baseline FHR and uterine activity recorded
- Contractions induced via:
- Nipple stimulation (preferred), or
- IV oxytocin administration (titrated)
- Goal: Achieve 3 contractions in 10 minutes
- FHR evaluated during and after contractions
- Test continues until adequate contractions or concerning patterns observed
- Also called Oxytocin Challenge Test (OCT) when oxytocin is used
Contraindications
- Absolute contraindications:
- Previous classical cesarean section
- Placenta previa
- Premature rupture of membranes
- Preterm labor
- Multiple gestation
- Relative contraindications:
- History of preterm labor
- Incompetent cervix
- Previous low transverse cesarean
| Result | Criteria | Interpretation |
|---|---|---|
| Negative (reassuring) | No late decelerations with adequate contractions | Indicates adequate uteroplacental reserve; fetus likely to tolerate labor well |
| Positive (non-reassuring) | Consistent late decelerations with ≥50% of contractions | Suggests uteroplacental insufficiency; fetus may not tolerate labor |
| Equivocal | Inconsistent late decelerations or significant variable decelerations | Unclear significance; may require repeat testing or additional evaluation |
| Unsatisfactory | Unable to achieve adequate contractions or poor tracing quality | Test cannot be interpreted; consider alternative testing methods |
Nursing Implications
- Verify that no contraindications exist before beginning the test
- Maintain IV access when oxytocin is used
- Position patient in semi-Fowler’s or left lateral position
- Monitor maternal vital signs throughout the procedure
- Have tocolytics available if hyperstimulation occurs
- Document frequency, duration, and intensity of contractions
- Discontinue test if concerning FHR patterns develop
- Prepare for possible delivery if positive result in term pregnancy
Memory Aid: CST-LATE
Remember key points about CST with CST-LATE:
- Contractions induce stress
- Stimulation via nipple or oxytocin
- Three contractions in 10 minutes needed
- Late decelerations are concerning
- Avoid in high-risk situations (contraindications)
- Tocolytics should be readily available
- Evaluate uteroplacental sufficiency
Doppler Ultrasound Assessment
Overview
Doppler ultrasound assessment evaluates blood flow through fetal and maternal vessels. It’s particularly valuable for detecting placental insufficiency and predicting adverse outcomes in high-risk pregnancies, especially those complicated by fetal growth restriction.
Vessels Commonly Assessed
- Umbilical artery (UA): Reflects placental resistance
- Middle cerebral artery (MCA): Detects brain-sparing effect
- Ductus venosus (DV): Assesses cardiac function and advanced compromise
- Uterine arteries: Evaluates maternal perfusion to placenta
Key Indications
- Suspected intrauterine growth restriction
- Preeclampsia
- Previous stillbirth
- Monochorionic twin pregnancy
- Abnormal placental appearance
- Diabetes with vascular disease
- Abnormal serum markers
| Vessel | Normal Finding | Abnormal Finding | Clinical Significance |
|---|---|---|---|
| Umbilical Artery | Forward flow throughout cardiac cycle; decreasing resistance with advancing gestation | Increased resistance, absent or reversed end-diastolic flow (AREDF) | AREDF indicates severe placental insufficiency and high risk of adverse outcome |
| Middle Cerebral Artery | Higher resistance than umbilical artery | Decreased resistance (brain-sparing effect) | Indicates fetal redistribution of blood flow to vital organs due to hypoxemia |
| Cerebroplacental Ratio (CPR) | MCA PI / UA PI > 1.0 | CPR < 1.0 | Better predictor of adverse outcome than individual vessel indices |
| Ductus Venosus | Forward flow throughout cardiac cycle | Absent or reversed a-wave | Late sign of fetal compromise; indicates cardiac decompensation |
Nursing Implications
- Explain the purpose and non-invasive nature of the test to reduce anxiety
- Position patient comfortably for optimal imaging
- Understand that abnormal Doppler findings may necessitate increased surveillance
- Prepare for possible hospitalization or delivery if severe abnormalities found
- Document Doppler results in the patient’s record
- Provide emotional support, as abnormal results can cause significant stress
Memory Aid: DOPE
Remember key aspects of Doppler assessment with DOPE:
- Doppler measures blood flow velocities
- Observe for absent/reversed end-diastolic flow (serious concern)
- Placental insufficiency detected early
- Evaluate multiple vessels for comprehensive assessment
Best Practices & Recent Updates
Best Practice #1: Individualized Testing Protocols
Testing frequency and methods should be individualized based on maternal risk factors, gestational age, and previous test results. New guidelines emphasize that a one-size-fits-all approach to antenatal testing is suboptimal.
Implementation: Develop patient-specific testing schedules based on risk stratification rather than applying universal protocols.
Best Practice #2: Integrated Assessment Approach
Combining multiple testing modalities (e.g., NST plus Doppler or modified BPP) provides more comprehensive information than single tests alone. Recent research supports this integrated approach for high-risk pregnancies.
Implementation: Use complementary testing methods that assess different aspects of fetal well-being rather than repeated application of a single test type.
Best Practice #3: Standardized Interpretation Criteria
Adopting consistent, evidence-based interpretation criteria for fetal testing reduces inter-observer variability and improves outcomes. Recent updates emphasize standardized approaches to test interpretation.
Implementation: Implement clear institutional protocols for test interpretation and management algorithms based on test results.
Recent Nursing Updates in Third Trimester Screening
- Telehealth Monitoring: Remote NST monitoring programs allow high-risk patients to perform testing at home with nursing oversight, reducing hospital visits while maintaining surveillance.
- Patient Education Tools: Development of standardized educational materials and visual aids to improve patient understanding of test purposes and procedures.
- Cerebroplacental Ratio (CPR): Increasing emphasis on CPR as a more sensitive predictor of adverse outcomes than individual vessel assessment in Doppler studies.
- Group B Strep Protocol Updates: Revised CDC guidelines for management of patients with penicillin allergies and updated antibiotic regimens.
- Automated FHR Analysis: Computer-assisted interpretation of fetal heart rate patterns to improve consistency in NST interpretation.
Summary of Third Trimester Screening
| Test | Timing | Primary Purpose | Key Nursing Considerations |
|---|---|---|---|
| Group B Strep Culture | 36-38 weeks | Detect maternal GBS colonization | Education about GBS transmission and intrapartum prophylaxis |
| Glucose Screening | If missed in 2nd trimester | Screen for gestational diabetes | Ensure proper timing of blood draws; patient preparation |
| Non-Stress Test | As indicated, typically weekly or biweekly | Assess fetal heart rate response to movement | Proper positioning; accurate documentation of movements |
| Biophysical Profile | As indicated, typically weekly | Comprehensive assessment of fetal well-being | Explanation of components; preparation for possible intervention |
| Contraction Stress Test | As indicated, less common | Assess fetal response to contractions | Screen for contraindications; monitor for hyperstimulation |
| Doppler Ultrasound | Serial assessment as indicated | Evaluate blood flow in fetal and maternal vessels | Understanding significance of findings; emotional support |
Memory Aid: TESTS
Remember the key third trimester tests with TESTS:
- Tracking growth with serial ultrasounds
- Evaluating heart rate patterns with NST
- Screening for Group B Strep
- Testing biophysical parameters with BPP
- Surveying blood flow with Doppler studies
These comprehensive notes on third trimester tests and screening are designed to help nursing students understand the importance, methodology, and interpretation of various antenatal assessment tools. Remember that testing protocols may vary by institution and should follow current evidence-based guidelines.