Prenatal Diagnostic Tests - Study Notes

Overview: Prenatal Genetic Screening and Diagnostic Testing

  • Distinguish between screening (risk assessment) and diagnostic testing (confirmation of abnormalities).
  • High-risk indicators for prenatal genetic concerns include:
    • Age > 35 years
    • History of previous losses
    • Family/genetic history
  • Purpose of screening: provide baseline information and accurate gestational age; assess risk without invasive procedures.
  • Purpose of diagnostic testing: confirm or rule out genetic problems when screening is abnormal or risk factors are present; invasive procedures are required.
  • General framework:
    • Screening tests are non-invasive and offered to all pregnancies.
    • Diagnostic tests are invasive and offered after abnormal screening results or when risk factors exist.
  • Important procedural note: after invasive testing (e.g., CVS), Rh status matters (RhoGAM for Rh-negative individuals).
  • Helpful resources referenced in training:
    • How to decide about prenatal genetic testing (video link provided in course materials).

Types of Prenatal Genetic Tests: Screening vs Diagnostic

  • FOR SCREENING (non-invasive, offered to all women):
    • 1) Maternal blood serum screening
    • 2) Prenatal ultrasounds
    • 3) Non-invasive prenatal testing (NIPT)
  • FOR DIAGNOSTING (invasive, after abnormal screening or known risk factors):
    • 4) Chorionic villus sampling (CVS)
    • 5) Amniocentesis
  • Notes:
    • CVS provides earlier diagnostic information (10–13 weeks gestation).
    • Amniocentesis is typically performed in the second trimester (15–20 weeks) but can be done later for specific assessments.

Types of Prenatal Tests (Overview)

  • Ultrasound
  • Doppler ultrasound blood flow assessment
  • Alpha-fetoprotein (AFP) screening
  • Multiple marker screening (maternal serum markers)
  • Chorionic villus sampling (CVS)
  • Amniocentesis
  • Percutaneous umbilical blood sampling (PUBS)
  • Antepartum fetal surveillance
    • Nonstress test (NST)
    • Vibroacoustic stimulation test
    • Contraction stress test (CST)
  • Biophysical profile (BPP)
  • Modified biophysical profile
  • Maternal assessment of fetal movement

Ultrasound

  • Uses:
    • Determine pregnancy presence and location
    • Assess multifetal gestation
    • Establish gestational age
    • Confirm viability
    • Identify fetal and placental abnormalities
  • Additional considerations:
    • Ultrasound can aid in emotional support for parents (visualization, reassurance)
    • Not always possible to determine fetal sex

Doppler Ultrasound Blood Flow Assessment

  • Method: Doppler ultrasound assesses blood flow via Doppler shift; used in various pregnancy complications.
  • Common indications:
    • Hypertension
    • Fetal growth restriction (FGR)
    • Placental insufficiency
  • Quantitative measure: systolic to diastolic ratio (S/D ratio)

Alpha-Fetoprotein (AFP) Screening

  • Purpose: maternal serum AFP to screen for open body wall defects (neural tube defects) and chromosomal anomalies (trisomy 21).
  • Timing: ideally between 16 and 18 weeks gestation.
  • Specimen: requires only a blood draw.
  • Interpretation: AFP screening is a first-step in screening; abnormal results require follow-up diagnostics.

Multiple Marker Screening (MMS)

  • Markers included:
    • Human chorionic gonadotropin (hCG)
    • Unconjugated estriol
  • Formats: Triple or Quad screen (Quad includes inhibin A as an additional marker in some protocols).
  • Purpose: increases detection of trisomy 18 and trisomy 21.
  • Next steps if positive: amniocentesis is recommended for diagnostic confirmation.

Chorionic Villus Sampling (CVS)

  • Purpose: obtain placental villi for genetic, metabolic, or DNA analysis; can detect chromosomal defects and other abnormalities.
  • Timing: performed between 10 and 13 weeks of gestation.
  • Methods: transcervical or transabdominal sampling.
  • Post-procedure care:
    • Rh status assessment; RhoGAM for Rh-negative individuals when indicated
    • After the procedure, assess fetal heart motion and maternal vital signs
    • Report heavy bleeding, passage of tissue, clots, or amniotic fluid; rest at home for several hours
  • Risks: fetal loss and other procedure-related complications (not exhaustive in notes).

Amniocentesis

  • Purpose: aspiration of amniotic fluid for fetal evaluation.
  • Timing: commonly performed in the second trimester (15–20 weeks); third-trimester amniocentesis can assess fetal lung maturity or hemolytic disease.
  • Indications for second-trimester amniocentesis (examples):
    • Maternal age ≥ 35 years
    • Chromosomal abnormality in a close family member
    • Gender determination for X-linked disorders if maternal carrier status is suspected
    • Birth of a previous infant with a chromosomal abnormality
    • Pregnancy after multiple spontaneous abortions
    • Unexplained elevation of maternal fetal alpha-fetoprotein (MSAFP)
    • Maternal Rh sensitization with Rh-negative mother and Rh-positive fetus

Percutaneous Umbilical Blood Sampling (PUBS)

  • Also called cordocentesis.
  • What it entails: aspiration of fetal blood from the umbilical cord near the placenta for prenatal diagnosis or therapy.
  • Uses: detect fetal blood disorders, acid–base imbalance, infection, or fetal genetic disease.
  • Notes: PUBS is infrequently needed to determine karyotype since many tests can be performed on fetal cells in amniotic fluid.
  • Potential complications: fetal loss, infection, fetal bradycardia, cord laceration, cord hematoma, thrombosis, thromboembolism, preterm labor, and PROM.

Monitoring Fetal Well-Being: Non-Stress Test (NST)

  • What NST assesses: fetal heart rate response to fetal movement.
  • Reactive NST (healthy): two accelerations of the fetal heart rate (≥ 15 beats per minute above baseline) lasting ≥ 15 seconds, occurring after movement within 20 minutes. For preterm (
  • Nonreactive NST: no accelerations or minimum fetal movements; or minimal baseline variability (< 6 bpm) across the testing period.
  • Next steps if nonreactive: schedule an additional fetal assessment, such as a Biophysical Profile (BPP).

Biophysical Profile (BPP)

  • Purpose: composite assessment of fetal well-being combining NST with ultrasound-based measurements.
  • Five components included in BPP:
    • Fetal reactivity (NST)
    • Fetal breathing movements (ultrasound)
    • Fetal body movements (ultrasound)
    • Fetal tone (ultrasound)
    • Amniotic fluid volume (ultrasound)
  • Scoring and interpretation:
    • Normal score: 8 to 10
    • Suspicious: 6
    • Abnormal/jeopardy: 4 or less
  • Normal and abnormal criteria (summary):
    • Fetal breathing movements: normal = 2 points; ≥ 30 seconds of breathing within 30 minutes
    • Fetal movement: normal = 2 points; ≥ 3 discrete body/limb movements within 30 minutes
    • Fetal tone: normal = 2 points; at least one episode of active extension/flexion of a limb or hand opening/closing within 30 minutes
    • Amniotic fluid volume: normal = 2 points; deepest vertical pocket ≥ 2 cm
    • NST: normal = 2 points; reactive (≥ 2 accelerations of 15 bpm for ≥ 15 seconds within 20 minutes); nonreactive = 0 points
  • Biophysical score interpretations:
    • 8–10: normal, fetus doing well
    • 6: suspicious
    • 4 or less: fetus potentially jeopardized

Antepartum Fetal Surveillance

  • Contraction Stress Test (CST) – historically used but no longer performed in some local hospitals.
  • Alternative: Oxytocin challenge test or nipple stimulation with low-dose oxytocin.
  • Purpose: evaluate fetal heart response to uterine contractions (decreased placental blood flow during contractions).
  • Contraindications: cannot be performed if stimulated uterine contractions are contraindicated.

Effects of Hypoxemia and Fetal Acidosis (Conceptual Chart)

  • Hypoxemia progression (late signs of fetal compromise):
    • Late decelerations appear first
    • Accelerations diminish next
    • Fetal breathing movements may stop
    • Fetal movement ceases
    • Fetal tone becomes absent (advanced compromise)
  • Abbreviated representation: these are indicators of worsening fetal oxygenation status and acidosis risk

Maternal Assessment of Fetal Movement (Kick Counts)

  • Initiation: typically starts around 28 weeks gestation
  • Method: mother counts fetal movements; several counting methods exist
  • Benchmark: at least 10 fetal movements within 1 hour
  • Action: inform provider if movements are consistently decreased or abnormal patterns emerge

Assessment of the Patient Undergoing Diagnostic Testing

  • Key demographic and clinical data to collect:
    • Gravida, para, living children, gestation
    • Maternal health problems
    • Current obstetric problems
    • Prior obstetric problems
    • History of substance abuse (including alcohol and tobacco)
  • Patient education and psychosocial considerations:
    • Patient understanding of the test and reasons for it
    • Knowledge of surveillance regimens for additional testing
    • Patient’s emotional response to testing
    • Patient and partner expectations of the test

Practice Questions (Key Scenarios and Answers)

  • Question 1 (Chorionic Villus Sampling): After CVS, which action should the nurse take immediately?

    • Options: A) Instruct fluid avoidance for 24 hours; B) Position on left side for 1 hour; C) Monitor fetal heart rate and maternal vital signs; D) Administer RhoGAM to all Rh-negative clients
    • Correct answer: C
    • Rationale: After CVS, monitor fetal heart tones and maternal vital signs. RhoGAM is given only if the client is Rh-negative.

  • Question 2 (MSAFP result): An abnormal maternal serum AFP test at 17 weeks indicates what next step?

    • Options: A) Result confirms neural tube defect; B) Repeat test in 4 weeks to confirm; C) Abnormal result suggests further testing such as amniocentesis may be needed; D) Schedule C-section at 37 weeks
    • Correct answer: C
    • Rationale: Abnormal MS-AFP is a screening result, not diagnostic; it indicates the need for additional testing (often amniocentesis) to determine neural tube defects or chromosomal anomalies.

  • Question 3 (NST interpretation at 35 weeks): A fetal heart rate with two accelerations of at least 15 bpm lasting 15 seconds in a 20-minute NST indicates:

    • Options: A) Reactive — fetal well-being is indicated; B) Nonreactive — further evaluation is needed; C) Suspicious — repeat NST in 24 hours; D) Inconclusive — perform CST
    • Correct answer: A
    • Rationale: A reactive NST at ≥32 weeks includes two accelerations of 15 bpm lasting at least 15 seconds within 20 minutes, indicating fetal well-being.

Connections and Practical Implications

  • Real-world relevance:
    • Screening tests identify pregnancies that require closer follow-up or diagnostic confirmation.
    • Diagnostic tests (CVS, amniocentesis) provide definitive information but carry procedure-related risks; decisions should balance benefits and risks with patient values.
  • Ethical and emotional considerations:
    • Counseling around test results, possible outcomes, and subsequent choices (e.g., further testing, pregnancy management) is essential.
  • Foundational links:
    • These principles align with obstetric care foundations: patient autonomy, informed consent, risk stratification, and balancing nonmaleficence with beneficence.

Key Formulas, Thresholds, and Numerical References (LaTeX)

  • NST acceleration thresholds (after 32 weeks):
    • Two accelerations: at least 15\,\text{bpm} for at least 15\,\text{s} within 20\,\text{min}
  • NST acceleration thresholds (before 32 weeks):
    • At least 10\,\text{bpm} for 10\,\text{s}
  • Biophysical Profile scoring: normal range = 8-10, suspicious = 6, abnormal/jeopardy = 4\text{ or less}
  • Amniotic fluid assessment in BPP: deepest vertical pocket ≥ 2\,\text{cm} considered normal for fluid volume
  • Fetal breathing movements: duration threshold ≥ 30\,\text{s} within 30\,\text{min}
  • Fetal movements: ≥ 3 discrete body or limb movements within 30\,\text{min}
  • Fetal tone: active extension/flexion within 30\,\text{min}

References to Course Materials

  • CVS: timing, procedure, post-procedure care, and Rh immunoglobulin administration
  • Amniocentesis: timing by trimester, indications for second-trimester testing, and use in lung maturity assessment
  • NST and BPP: interpretation criteria and management pathways when results are reactive or nonreactive
  • Practical questions mirror standard nursing exam-style scenarios and emphasize interpretation of test results and appropriate follow-up actions