13.3 Prenatal Testing during the Third Trimester

Biophysical Profile

A biophysical profile may be recommended if other prenatal screenings came back nonreassuring, like a nonreactive stress test or complaints of decreased fetal movement (Sapoval, 2023). A biophysical profile (BPP) is a specialized noninvasive test that checks the fetus’s growth and well-being. This test uses a scoring system to evaluate fetal well-being in the following areas:

1. fetal heart rate pattern per NST
2. fetal breathing movements
3. fetal body movements
4. fetal muscle tone
5. amount of amniotic fluid

Each of the five areas is given a score of zero or two for 10 total possible points (ACOG, 2019a) (Table 13.5).

A high-risk obstetric and fetal condition may also require a BPP as part of antepartum testing for a variety of fetal conditions found in Table 13.6. This study is recommended to start at 32 weeks’ gestation for most fetal and maternal conditions but can be done earlier for multiple or severe conditions (Sapoval, 2023).

A BPP is an indicator of fetal well-being, and a score of 8 or 10 is considered normal and indicates a decreased risk of fetal asphyxia within 1 week. A score of 6 is considered abnormal and should be repeated within 24 hours. A score of 2 or 4 is not reassuring, and the provider may recommend inducing labor or scheduling a cesarean delivery for the pregnant person. A score of zero indicates impending fetal asphyxia, and an emergent cesarean delivery at a hospital with a neonatal intensive care unit is recommended (Sapoval, 2023). A BPP can also diagnose oligohydramnios or polyhydramnios. If oligohydramnios is diagnosed, the study should be repeated within 24 hours, or the provider may recommend the pregnant person be induced if at term.

The nurse’s role involves educating the pregnant person about the BPP. The nurse can help prepare the pregnant person by explaining why the test is recommended and what to expect during the screening. The nurse will perform the NST and reinforce the results and plan of care developed by the health-care provider.

Amniotic Fluid Index (AFI)

The amniotic fluid index (AFI) is a standardized way to assess the sufficiency of the amniotic fluid quantity in pregnancy and is obtained via ultrasound. This test can be done on pregnant persons who are at 24 weeks’ gestation or greater with a singleton pregnancy. This test is also part of the biophysical profile and is an indicator of fetal well-being. A normal AFI is 5 cm to 25 cm. A normal pocket of amniotic fluid is greater than 2 cm (Lord et al., 2023).

An AFI test may be recommended if there are concerns of polyhydramnios, which is too much amniotic fluid, or oligohydramnios, which is not enough amniotic fluid (Lord et al., 2023). Oligohydramnios can occur if the pregnant person’s amniotic membrane has ruptured and amniotic fluid is leaking. It can also occur due to kidney problems in the fetus. If the fetus receives inadequate nutrients and oxygen from the placenta, blood will be shunted away from the fetal kidneys. This decreases the glomerular filtration rate (GFR) and results in decreased urinary output. This causes a decrease in amniotic fluid due to the decreased urine production by the fetal kidneys (Lord et al., 2023). Persistently low levels of amniotic fluid may be associated with a birth defect of the fetus.

Polyhydramnios may have no definite cause. The normal fetus constantly swallows amniotic fluid and urinates to create more. If the fetus is not able to swallow normal amounts of amniotic fluid, it can lead to polyhydramnios. This could indicate gastrointestinal malformations, fetal neurologic problems like anencephaly, or mechanical obstruction of the esophagus. Increased amniotic fluid production can also result from polyuria, which can occur with uncontrolled maternal diabetes. This may be associated with fetal macrosomia (Lord et al., 2023).

If an AFI screening is ordered, the nurse can help prepare the pregnant person by explaining why the test is being recommended, what to expect during the test, when the results will be available, and what the results mean.

Fetal Movement Count (FMC)

The pregnant person should feel at least 10 fetal movements within 2 hours and will likely feel more than that. If the pregnant person notices that they are feeling fewer fetal movements than normal, they should talk to their health-care provider about it. The health-care provider may recommend keeping track of the fetus’s movements by doing a fetal movement count or “kick count,” a test that can be performed at home by the pregnant person (ACOG, 2019a). Table 13.7 provides instructions on how to perform fetal movement counts.

Nurses can help educate pregnant persons on how to perform fetal movement counts. The best time to perform FMC is when the fetus is most active. If the fetus does not have a predictable movement pattern, completing the FMC after eating dinner is a good time. To perform this test, the nurse should instruct the pregnant person to lie down on their side or relax in a comfortable chair. Take note of the time. Pay attention only to the movements of the fetus. Count any movement felt, except for hiccups. Any twist, kick, or turn counts as one movement. After 10 movements are felt, check the time and record how many minutes it took to feel 10 movements (Kaiser Permanente, n.d).

If the fetus does not kick or move within 1 hour, the pregnant person should be instructed to eat or drink something like fruit or juice, lie on their left side, or walk around for 5 minutes. If the fetus has not moved 10 times by the end of the 2 hours or has a sudden decrease in normal activity, the pregnant person should be instructed to notify their health-care provider right away (Srisuro, 2022).

Vibroacoustic Stimulation

The noninvasive technique that uses vibratory and sound stimulation to determine fetal well-being is called vibroacoustic stimulation (VAS) (Figure 13.9). VAS is used when an NST is nonreactive to stimulate FHR accelerations (ACOG, 2021c). During this test, the vibroacoustic stimulator is placed on the pregnant person’s abdomen near the location of the fetal head. The stimulator produces a sound at a predetermined level for 1 to 3 seconds. The expected response by the fetus is the startle reflex, causing an acceleration of the fetal heart rate. The acceleration is linked with fetal well-being. The absence of the startle reflex in the fetus is nonreassuring and requires further testing, such as a contraction stress test or BPP. The vibroacoustic stimulation test allows providers to determine if the fetus may require delivery (East et al., 2013).

Figure 13.9 Vibroacoustic stimulation device A vibroacoustic stimulator is placed on the enlarged uterus. The fetus is expected to react to the sound emitted from the stimulator by moving. Fetal movement is expected to cause an acceleration of the fetal heart rate. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

When a vibroacoustic stimulation test is ordered, the nurse will need to educate the pregnant person on what the test is, why it is being ordered, and what to expect during the test. After the test is performed, the nurse will inform the provider of the results, then explain the results to the pregnant person and inform the pregnant person if additional interventions have been prescribed by the health-care provider.

Fetal Nonstress Test

A nonstress test (NST) is a noninvasive test for fetal well-being that provides a graphic of the fetal heart pattern in relation to the movement of the fetus (Figure 13.10). The external fetal and uterine contraction monitor is used to perform an NST. This test places no stress on the fetus. A nonstress test can be performed starting at 28 weeks’ gestation (U.S. National Library of Medicine, 2021a).

Figure 13.10 Reactive NST The FHR accelerations are linked with the fetal movements in the uterine contraction tracing. In this illustration, the FHR increased at least 15 beats per minute (bpm) above the baseline of 130 two or more times in this 10-minute tracing. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

A health-care provider may recommend an NST for one of the following reasons:

• There is decreased fetal movement.
• Pregnant person is past the estimated due date.
• Pregnant person has a chronic medical condition such as high blood pressure, diabetes, heart disease, or a clotting disorder.
• Pregnant person has a pregnancy-related complication such as gestational diabetes, hypertension in pregnancy, or placental abnormality.
• Pregnant person had a complication in a previous pregnancy.
• Pregnant person is having multiples.

If the fetal heart rate increases by at least 15 bpm above the baseline for 15 seconds for a fetus at 32 weeks or greater, or 10 bpm for 10 seconds in the fetus less than 32 weeks, two or more times during a 20-minute testing period, it is considered a reactive NST and is a reassuring result. If there are insufficient accelerations of the fetal heart rate within the 20- to 40-minute window or the accelerations do not meet the required criteria, the NST is nonreactive. However, a nonreactive result does not necessarily indicate a health problem. The fetus may have been sleeping and not easily awakened. Certain medications taken during pregnancy may also cause a nonreactive result. The health-care provider will review the results and determine if additional testing is needed to find out if there is a cause for concern (U.S. National Library of Medicine, 2021a).

The nurse can help prepare the pregnant person for this test by explaining why the test is recommended and what happens during the test. The test may be done in the provider’s office or at the hospital. The pregnant person will lie on a reclining chair or an exam table when the test is performed in the office or clinic. The nurse will attach two devices around the pregnant person’s abdomen. One will measure the fetal heart rate, and the other will record the pregnant person’s uterine activity. (See Chapter 16 Electronic Fetal and Uterine Contraction Monitoring for more information on external fetal monitoring.) The pregnant person will be instructed to press a button on a cable attached to the fetal monitor each time they feel the fetus move. If the test is reactive, the nurse will inform the health-care provider and provide the pregnant person with further education and instructions as prescribed.

If the fetus isn’t active or moving during the test, it may indicate the fetus is sleeping. The nurse will ask the pregnant person to have a snack or sugary drink to wake the fetus. After 40 minutes, if the test remains nonreactive, the nurse will notify the health-care provider and a plan for further testing, often a BPP, is relayed to the pregnant person by the nurse.

Contraction Stress Test

A contraction stress test (CST) may be recommended if the NST is nonreactive, or unclear, when ultrasound for a BPP is not available. The CST is not widely used anymore due to the availability of a BPP, which is less invasive. A contraction stress test evaluates the response of the fetal heart rate to uterine contractions. During contractions, there is a decrease in the oxygenation of the fetus. If the fetus lacks a reserve of oxygen (demonstrating uteroplacental insufficiency), then a nonreassuring FHR pattern is noted during contractions (ACOG, 2021c).

A contraction stress test allows the provider to see how the fetal heart rate reacts when the uterus contracts. During this test, the external fetal and contraction monitor is applied. (See Chapter 16 Electronic Fetal and Uterine Contraction Monitoring for more information on external fetal monitoring.) The health-care provider will order oxytocin (Pitocin) or have the pregnant person perform nipple stimulation to make their uterus contract. The expected uterine contraction pattern for a CST is at least three contractions lasting for a minimum of 40 seconds each within a 10-minute period. If the patient is already experiencing sufficient spontaneous contractions, there is no need for uterine stimulation. However, if there are fewer than three contractions of 40 seconds each within 10 minutes, nipple stimulation or intravenous oxytocin will be used to induce contractions. A spontaneous CST can be considered if the necessary number and strength of contractions are achieved within the 10-minute time frame (ACOG, 2019a).

The CST test results are divided into several categories:

• Negative: This means there are no late or significant variable decelerations (Figure 13.11).
• Positive: Late decelerations occur after 50 percent or more of contractions, even if the contraction frequency is less than three in 10 minutes (Figure 13.12).
• Equivocal-suspicious: The test shows intermittent late decelerations or significant variable decelerations.
• Equivocal: This means there are FHR decelerations present during contractions that occur more frequently than every 2 minutes or last longer than 90 seconds.
• Unsatisfactory: The tracing shows fewer than three contractions in 10 minutes or is otherwise uninterpretable. (ACOG, 2019a). (See Chapter 16 Electronic Fetal and Uterine Contraction Monitoring for description of late decelerations of the fetal heart rate.)

Figure 13.11 Negative CST In this illustration, the FHR baseline is 130 and is at the top of the graph. Contractions are recorded on the bottom. This strip shows that with each contraction, or peak, at the bottom, the heart rate remains stable, which makes it a negative result. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Figure 13.12 Positive CST In this illustration, the FHR baseline is 130. The fetal heart rate is at the top and the contractions are on the bottom. This strip shows a dip in the fetal heart rate just after a contraction occurs, known as a late deceleration. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Fetal surveillance with a CST is done to identify the fetus’s risk of hypoxic injury or death so that appropriate intervention can be done to prevent an adverse outcome if possible. It can also identify normally oxygenated fetuses to allow pregnancy to continue safely and avoid unnecessary intervention (ACOG, 2021d).

The nurse can help prepare the pregnant person for a CST by explaining why the test is recommended, answering any questions, and describing what will happen during the test. Prior to the CST, the nurse will need to get a baseline tracing, and the nurse will monitor the tracing afterwards while a plan of care is being determined. The provider will review the results directly after the test is completed and discuss with the pregnant person if continuing the pregnancy, further testing, or delivery is the best plan.