Maternal-Newborn Nursing · Prenatal Testing
Prenatal Testing during the Third Trimester
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In 30 seconds
The third trimester — roughly week 28 through birth — is the final stretch of prenatal care, and its testing serves a different job than earlier testing. Early testing is mostly screening for anomalies and risk; third-trimester testing is mostly surveillance: asking whether the fetus is thriving right now and whether the pregnant person's body is staying healthy as birth approaches. The tools covered here — Fetal movement counting The pregnant person tracking fetal movement per provider instructions Full entry →, the Nonstress test (NST) External monitoring of fetal heart rate and uterine activity, watching for accelerations Full entry →, Biophysical profile (BPP) NST plus ultrasound scoring of fetal breathing, movement, tone, and fluid Full entry →, Contraction stress test (CST) Monitoring fetal heart rate while mild contractions are provoked Full entry →, ultrasound, and Group B Streptococcus (GBS) screening — answer those questions. For nurses, the topic is also about process: preparing the person and getting results to the right clinician.
Why this matters
Surveillance exists because some problems begin late or worsen as the placenta ages — reduced fetal movement, decreasing amniotic fluid, GBS colonization, anemia. Catching them lets the team adjust the plan before a crisis. The nurse often schedules the test, explains it, positions the person, watches the tracing, and documents it, so knowing what each test measures is a patient-safety issue — and a scope-of-practice one, since many results are interpreted by providers.
The college version
Core Concepts
The purpose shift: from screening to surveillance
Early testing hunts for problems in development. By the third trimester the fetus is developed; the question is whether the environment still supports it. Surveillance therefore focuses on what sustains the fetus: oxygen delivery through the placenta and adequate amniotic fluid — hence so many tests are variations on "watch the heart-rate response and check the fluid."
Fetal movement counting ("kick counts")
The simplest surveillance tool is done at home: the pregnant person tracks how often the fetus moves, since a fetus conserving oxygen moves less. Instructions vary by provider and institution, so a key nursing task is teaching the specific instructions and telling the person who to call if movement seems low. A reported decrease is always taken seriously and passed to the provider.
Nonstress test (NST)
The most common clinic-based surveillance test. "Nonstress" means no contraction is applied: an external Doppler records the fetal heart rate while a tocodynamometer records uterine activity, usually about 20–30 minutes. The test looks for accelerations — brief rises in fetal heart rate, typically with fetal movement — which signal an intact nervous system and oxygenation. Interpretation belongs to the provider or an authorized clinician; the nurse prepares, educates, positions (often with a left lateral tilt), and documents.
Biophysical profile (BPP) and modified BPP
A BPP combines the NST with ultrasound scoring of four fetal behaviors: breathing, gross body movements, tone, and amniotic fluid volume. The summed score helps the provider judge well-being. The modified BPP is shorter — the NST plus a fluid check only — and is often used for frequent surveillance.
Contraction stress test (CST)
The CST does the opposite of the NST: it deliberately provokes mild contractions — via nipple stimulation or medication ordered by the provider — while monitoring the fetal heart rate, to see how the fetus tolerates labor-type stress. It is performed where emergency care is immediately available and is less common than in past decades.
Ultrasound: growth, fluid, placenta, and position
Third-trimester ultrasound is used selectively: checking fetal growth, amniotic fluid volume, placental location (for example, whether the placenta covers the cervix), and Fetal presentation Which fetal part is at the cervix (head-down, breech, transverse) Full entry → (head-down versus breech). In high-risk pregnancies, Doppler ultrasound of umbilical blood flow may evaluate placental function. Results feed directly into birth planning.
Group B Streptococcus (GBS) screening
GBS is a bacterium that can live harmlessly in the gastrointestinal or genital tract but can cause serious infection in a newborn exposed during vaginal birth. Because colonization comes and goes, screening is timed late in pregnancy — commonly near 36–37 weeks — using a vaginal-rectal swab. This is a widely published recommendation (ACOG and CDC), but guidelines and hospital protocols differ, so nurses should verify current guidance. A positive result means colonization; the provider plans intrapartum care.
Repeat laboratory screening and Rh antibody checks
Some laboratory tests are repeated because they can change: a complete blood count may be rechecked for anemia, urine for protein, and infection screenings may be repeated in some populations. For an Rh-negative pregnant person, antibody screening may be repeated to detect sensitization — antibodies against Rh-positive fetal cells. Which tests are repeated varies by history, region, and institution.
The nurse's role in third-trimester testing
The nursing work follows a similar arc: prepare (explain purpose, what the person will feel, how long it takes); perform or assist (position, monitor, document); and follow through (ensure results are communicated). Scope matters at every step: interpreting tracings and scoring profiles are provider responsibilities that vary by state law and institutional policy.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| NST | CST | The NST applies no stress; the CST deliberately provokes contractions |
| Kick counts | NST | Kick counts happen at home; the NST is a clinic-based electronic tracing |
| BPP | Routine ultrasound | A BPP combines the NST with scored fetal-behavior observations; a scan is imaging only |
| GBS screening | Urine infection screening | GBS is a vaginal-rectal swab for colonization, not a UTI test |
| "Non-reassuring" result | "The baby is in trouble" | Non-reassuring means review and further evaluation are needed — not a diagnosis |
Eli explains
The same idea, in plain words
Explain it like I’m 10
When a baby is almost ready to be born, the care team runs "check-up" tests to make sure the baby is doing fine inside. Some count how often the baby moves; others listen to the heartbeat to see it speed up when the baby wiggles. If a check-up shows something unusual, the team watches more closely or makes a plan to help.
Worked example
A pregnant person at 39 weeks calls the clinic and reports the baby "hasn't moved as much today." The triage nurse gathers details, checks the chart for risk factors, and relays the report to the provider, who orders a nonstress test. The nurse explains the test — two belts on the abdomen, about half an hour, nothing painful — helps the person into a comfortable left lateral tilt, and stays nearby while the tracing runs. The nurse watches the tracing for technical quality but does not label it reassuring or non-reassuring; the provider reviews it and discusses the plan. The tracing shows the expected accelerations, and the nurse documents the education and outcome before discharge. Most of the work was nursing work — triage, preparation, positioning, monitoring, documentation — while interpretation stayed with the provider.
Key takeaways
- Testing shifts from screening for anomalies to surveillance of fetal well-being and late-pregnancy risk.
- Kick counts are low-tech home surveillance; teach the person's specific instructions and take decreased movement seriously.
- The NST watches for fetal heart rate accelerations with movement — no contraction is applied.
- The BPP adds ultrasound-scored breathing, movement, tone, and fluid; the modified BPP pairs the NST with a fluid check only.
- The CST provokes mild contractions to test labor tolerance and needs an emergency-ready setting.
- Ultrasound checks growth, fluid, placental location, and presentation — results can change the birth plan.
- GBS screening is commonly timed near 36–37 weeks via vaginal-rectal swab; verify current guidance.
- Rh antibody screening may be repeated to detect sensitization; follow-up follows provider orders.
- Nurses prepare, educate, position, monitor, and document — they do not interpret tracings unless their role authorizes it.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
How and why does the purpose of testing change in the third trimester?
Show answer
Testing shifts from screening for developmental problems to surveillance of current fetal well-being and late-pregnancy risks.
What does the NST measure, and what does "nonstress" mean?
Show answer
The NST records fetal heart rate and uterine activity externally, looking for accelerations with fetal movement. "Nonstress" means no contraction is provoked.
What is the difference between a BPP and a modified BPP?
Show answer
A BPP combines the NST with ultrasound scoring of fetal breathing, movement, tone, and fluid; a modified BPP adds only a fluid check.
What is GBS screening Late-pregnancy vaginal-rectal swab for GBS colonization Full entry →, and why is its timing important?
Show answer
GBS screening is a vaginal-rectal swab timed late in pregnancy (commonly around 36–37 weeks) to detect colonization closest to birth; a positive result informs intrapartum management. Timing follows current recommendations.
List three nursing responsibilities during third-trimester testing; which judgments stay with the provider?
Show answer
Prepare and educate the person, position and monitor, and document results and teaching. Interpreting tracings or scores is a provider responsibility unless state law and facility policy authorize the nurse.
A pregnant person reports decreased fetal movement. What should the nurse do?
Show answer
Gather details, review the chart, and report promptly to the provider — decreased movement is a reportable concern.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Fetal movement counting
- The pregnant person tracking fetal movement per provider instructions
- Nonstress test (NST)
- External monitoring of fetal heart rate and uterine activity, watching for accelerations
- Acceleration
- A temporary rise in fetal heart rate, often with fetal movement
- Biophysical profile (BPP)
- NST plus ultrasound scoring of fetal breathing, movement, tone, and fluid
- Contraction stress test (CST)
- Monitoring fetal heart rate while mild contractions are provoked
- GBS screening
- Late-pregnancy vaginal-rectal swab for GBS colonization
- Fetal presentation
- Which fetal part is at the cervix (head-down, breech, transverse)
- Rh sensitization
- Rh-negative person making antibodies against Rh-positive fetal cells
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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