Unit 2- Prenatal Diagnostic Tests and Fetal Assessment

Prenatal Diagnostic Tests and Fetal Assessment Module Overview

  • This module focuses on common screening and diagnostic antepartum fetal testing.

Objectives

  • Coverage of the following prenatal diagnostic tests:

    • Ultrasound

    • Used to determine various fetal and placental conditions including:

      • Presence and location of pregnancy

      • Multifetal gestation

      • Gestational age

      • Confirmation of viability

      • Identification of fetal abnormalities

    • Color Doppler:

      • Blood flow assessment

    • Emotional responses are noted.

    • Note: Determining fetal sex is not always possible.

First Trimester Procedure

  • The woman is placed in the lithotomy position for a transvaginal ultrasound.

    • Transvaginal probe:

    • Encased in a disposable cover and coated with gel which provides lubrication and aids sound wave conduction.

    • The probe is inserted into the vagina.

Second Trimester Procedure

  • The mother is positioned as follows:

    • On her back with head and knees supported.

    • Elevated head and turned to one side with a wedge rolled blanket under one hip to prevent supine hypotension.

    • A gel-coated transducer is moved across the abdomen to obtain a clear image.

    • A full bladder may aid in displacing intestines and elevating the uterus for better visibility.

Purposes of Ultrasound

  • Confirmation of fetal viability.

  • Evaluation of fetal anatomy.

  • Estimation of gestational age.

  • Assessment of fetal growth progress.

  • Comparing growth progress in multifetal gestation.

Advantages of Ultrasound

  • Safe, clear visualization of the fetus.

  • Non-invasive and relatively comfortable for the patient.

  • Results are immediately available.

Disadvantages of Ultrasound

  • Cannot identify every fetal structural defect or defects.

  • Cost issues for uninsured patients.

  • Normal findings may occur that have unknown implications.

Alpha-Fetoprotein Screening (AFP)

  • AFP assessment can be performed on maternal serum or amniotic fluid, identifying:

    • Open body wall defects (neural tube defects).

    • Chromosomal anomalies (trisomy 21).

  • Ideal Timing: Performed between 16 and 18 weeks of gestation.

Advantages of Alpha-Fetoprotein Screening

  • Requires only a blood sample.

  • The least invasive and most economical procedure for screening open body wall defects.

  • Allows parents time to prepare for the birth of a potentially special needs infant.

Disadvantages of Alpha-Fetoprotein Screening

  • Must be viewed as a first step in screening.

  • The timing of the test can impose limitations.

Multiple Marker Screening

  • Includes two additional markers:

    • Human chorionic gonadotropin (hCG).

    • Unconjugated estriol.

  • Increases detection rates for trisomy 18 and 21.

  • If results are abnormal, amniocentesis is recommended.

Chorionic Villus Sampling (CVS)

  • Involves transcervical or transabdominal sampling to obtain villi.

  • Can be performed earlier than amniocentesis.

  • Provides information on:

    • Chromosomal defects.

    • Metabolic and DNA abnormalities.

  • Timing: Performed between 10 and 13 weeks of gestation.

Procedure for Chorionic Villus Sampling

  • Approaches available:

    • Transcervical: A flexible catheter is inserted through the cervix, and a sample of chorionic villi is aspirated.

    • Transabdominal: A needle is inserted through the abdominal and uterine walls to collect chorionic tissue.

Advantages of Chorionic Villus Sampling

  • Results are usually available quicker than amniocentesis.

  • Early performance in pregnancy.

  • May be less physically and emotionally traumatic than a later procedure, should the woman choose abortion.

Limitations of Chorionic Villus Sampling

  • Labor-intensive as maternal cells may be aspirated along with fetal cells.

Amniocentesis

  • Involves aspiration of amniotic fluid from the sac for analysis.

  • Timing:

    • Second trimester: Performed between 15 and 20 weeks for chromosomal or biochemical abnormalities.

    • Third trimester: Used to determine fetal lung maturity or hemolytic disease.

Procedure for Amniocentesis

  • The woman is positioned supine with a rolled towel.

  • Ultrasound is used to locate the fetus and placenta and identify large pockets of amniotic fluid for safe sampling.

  • Skin is treated with antiseptic.

  • Local anesthetic is injected into the skin.

  • A 3-4 inch, 20 or 21-gauge needle is inserted into the fluid pocket.

  • The first 1-2 mL is discarded to avoid contamination, and 20 mL of fluid is collected for analysis.

Advantages of Amniocentesis

  • Relatively safe procedure.

  • Capable of diagnosing many fetal anomalies and confirming fetal lung maturity.

  • Brief and relatively painless.

  • Associated with few complications.

Disadvantages of Amniocentesis

  • Major disadvantage is the timing of midtrimester amniocentesis (15-20 weeks).

Non-Stress Test (NST)

  • Assesses fetal well-being and oxygenation of the placenta

  • Evaluates if there are changes in the fetal heart rate with movement

  • Increase in fetal heart rate with movement = acceleration = good

  • Decrease in fetal heart rate with movement = deceleration = bad

  • This is a sign that the fetus will not tolerate labor

  • Method: Non-invasive; employs an external fetal heart rate monitor and tocometer on the mother’s abdomen.

    • The mother presses a button upon feeling fetal movement.

  • Gestational Age: Typically performed in the third trimester (after 28 weeks).

Indication for NST

  • Used in high-risk pregnancies, such as:

    • Maternal hypertension or diabetes.

    • Decreased fetal movement or post-term pregnancy.

Risk Factors for NST

  • No direct risks; safe for both mother and fetus.

Implications of NST Results

  • A reactive NST shows at least two accelerations of fetal heart rate in 20 minutes, indicating good oxygenation and neurological function.

  • A non-reactive result may require further testing (e.g., Biophysical Profile or Contraction Stress Test).

  • Disadvantage: False positive tests may occur in well-oxygenated term fetuses of 32 weeks gestation or more.

Contraction Stress Test (CST)

  • Preformed when the non-stress test is non-reactive

  • Oxytocin is administered to induce contractions and the baby is monitored to evaluate their response to contractions

  • Checking to see if the baby will tolerate labor, or show signs of stress

  • Method: External fetal monitoring during induced uterine contractions via oxytocin or nipple stimulation.

  • Gestational Age: Usually performed after 34 weeks to mitigate preterm labor risk.

Indication for CST

  • Utilized when NST is non-reactive or concerns regarding placental function and fetal oxygenation arise.

Risk Factors for CST

  • Invasive; can induce labor or excessive contractions (uterine hyperstimulation) leading to fetal distress or preterm labor.

  • Contraindicated in women at risk for preterm labor or with certain uterine or placental issues.

Implications of CST Results

  • A negative CST (no late decelerations) is reassuring.

  • A positive CST (late decelerations with contractions) suggests uteroplacental insufficiency, possibly indicating the need for delivery.

Biophysical Profile (BPP)

  • Assesses 5 main areas: body movements, muscle tone, breathing movements, amniotic fluid and fetal heartbeat (the NST)

  • Each area will get a score of 2 if normal finding, or a 0 if it is abnormal Higher score = better! 10/10 indicates the fetus is very healthy

  • Score of less than 8 indicates need for further evaluation/testing

  • Very low scores indicate fetus is showing multiple signs of distress and that early delivery should be considered

  • Method: Combines NST with ultrasound assessment of:

    • Fetal breathing.

    • Movement.

    • Tone.

    • Amniotic fluid volume (total of 5 parameters).

  • Gestational Age: Typically performed in the third trimester, especially after 32 weeks.

Indication for BPP

  • Used for high-risk pregnancies, non-reactive NST, or concerns about fetal hypoxia.

Risk Factors for BPP

  • Non-invasive; no direct risks to mother or fetus.

Implications of BPP Results

  • Each parameter scored as 0 or 2; total scores indicate:

    • 8-10: Normal.

    • 6: Equivocal.

    • 4 or less: Abnormal, indicating possible fetal compromise and need for intervention.

Maternal Assessment of Fetal Movement

  • Method: Mother counts fetal movements over a specific period (e.g., 10 movements in 2 hours).

  • Gestational Age: Can start as early as 28 weeks, but most beneficial in the third trimester.

  • Indication: Routine for all pregnancies, especially high-risk; decreased movement may indicate distress.

  • Risk Factors: Completely non-invasive; no inherent risks.

Implications of Maternal Assessment of Fetal Movement

  • Decreased fetal movement may prompt further assessments (NST, BPP).

  • A simple, cost-effective screening tool though less specific than other tests.

Clinical Reasoning and Nursing Process

  • Students are expected to:

    1. Compare and contrast different approaches to fetal assessment, considering:

    • Method of testing.

    • Gestational age for testing.

    • Indications.

    • Risk factors.

    • Implications.

    1. Discuss the purpose, procedure, advantages, and risks associated with each diagnostic procedure.

Learning Guide

  • Begin the module by reviewing the learning objectives and reading assigned textbook chapters.

  • Review associated lecture slides and attend in-person lessons.

  • Supplemental learning activities available at the bottom of the module.

  • Questions can be posted in the Discussion Board's Q & A section.

  • Read: Chapter 15 – pp. 279-291.