Maternal-Newborn Nursing · Prenatal Testing
Prenatal Testing during the Second Trimester
On this page 9 sections
In 30 seconds
The second trimester — roughly weeks 14 through 27 — is the busiest testing period of pregnancy. Three kinds of testing happen here:
- The Anatomy ultrasound Detailed ~18–22 week scan surveying fetal structures, placenta, and fluid Full entry → (~18–22 weeks), which surveys the fetus's developing structures.
- Maternal serum screening (the Quad screen Second-trimester blood screen measuring AFP, hCG, estriol, inhibin A Full entry →, ~15–22 weeks), which estimates risk for chromosomal conditions and for open neural tube defects — something the first-trimester screens do not assess.
- Diagnostic testing — most importantly Amniocentesis Diagnostic sampling of amniotic fluid (fetal cells) after ~15 weeks Full entry → (typically offered after ~15 weeks), which gives a definitive chromosomal answer when screening or ultrasound findings warrant it.
The second trimester also brings routine gestational diabetes screening (~24–28 weeks). And second-trimester results are frequently combined with first-trimester results (integrated or sequential screening) for the most accurate risk estimates — so the two topics in this chapter are a continuum, not competitors.
Why this matters
- The anatomy scan is the closest look at fetal structure most families get — it detects many structural anomalies, locates the placenta, measures amniotic fluid, and guides growth monitoring.
- The quad screen is the only common blood-screening approach that covers open neural tube defects (spina bifida, anencephaly) via its AFP marker.
- Amniocentesis gives definitive answers when screening or ultrasound raise concerns; knowing what it involves — and what to teach afterward — is a core nursing skill.
- Glucose screening catches gestational diabetes in time to manage it, reducing the risk of a very large baby, birth injury, and newborn low blood sugar.
- Nurses do the explaining: what each test is for, what a screen-positive means, what the procedure involves, and what symptoms to report afterward — so clear communication is patient safety.
The college version
Core Concepts
The anatomy ultrasound (~18–22 weeks)
The anatomy survey examines fetal structures systematically: head and brain, face, spine, heart (four chambers and great vessels), chest, abdomen and internal organs, kidneys and bladder, arms and legs, and sex organs (if the family wants to know). It also evaluates:
- Placental location — including whether the placenta is low-lying or covering the cervix (previa; see Chapter 12).
- Amniotic fluid volume — too little (oligohydramnios) or too much (polyhydramnios) can signal problems.
- Fetal growth — measurements used to estimate fetal size.
The scan is noninvasive and safe but has limits: some structures are hard to see depending on fetal position and gestational age, and not every anomaly is detectable prenatally. Uncertain findings usually prompt a follow-up scan or referral to a specialized fetal center — not immediate alarm. Frame the scan realistically: "normal anatomy scan" means "no problems found on this exam," not "guaranteed healthy baby."
Maternal serum screening: the quad screen
The quad screen is a blood test typically drawn ~15–22 weeks that measures four substances:
- AFP (alpha-fetoprotein) Fetal liver protein measurable in maternal blood Full entry → — a fetal liver protein; maternal blood levels screen for open neural tube defects and some other conditions.
- hCG, estriol, and inhibin A — pregnancy-related hormones/proteins whose patterns, combined with AFP and maternal age, contribute to the chromosomal risk estimate.
The result is a risk estimate, not a diagnosis. Unusual values prompt follow-up: repeat testing, detailed ultrasound, and possibly genetic counseling or diagnostic testing. As with all screening, a "positive" screen in a healthy pregnancy happens occasionally by design — the test is meant to catch as many real cases as possible.
AFP and neural tube defect screening
AFP deserves special attention because it is the marker that covers a condition no other routine blood screen covers. AFP levels change with gestational age, so accurate dating is essential — a dating error can produce a falsely elevated or low AFP. Elevated AFP is associated with open neural tube defects (spina bifida, anencephaly) and some other conditions (including multiple gestation and certain abdominal wall defects). Follow-up for an elevated AFP typically includes a detailed ultrasound of the spine and skull, and possibly amniocentesis to measure AFP in the amniotic fluid directly. Low AFP is part of the chromosomal risk pattern evaluated with the other markers.
Amniocentesis: the second-trimester diagnostic test
Amniocentesis obtains a small sample of amniotic fluid — which contains fetal cells — by passing a thin needle through the abdomen into the amniotic sac, guided by ultrasound. It is typically performed after ~15 weeks, when enough fluid has accumulated. The fetal cells are cultured and analyzed (karyotype and often chromosomal microarray) for a definitive chromosomal answer; amniotic fluid AFP can also be measured for neural tube defects. It is invasive, with a small risk of complications including pregnancy loss — which is why it is offered only after counseling and the decision belongs entirely to the pregnant person.
Nursing care around amniocentesis:
- Before: explain the procedure, what results can and cannot tell, and the small risks; support informed consent; confirm Rh status — an Rh-negative person receives anti-D immune globulin per protocol afterward to protect future pregnancies.
- During (assisting): support the pregnant person, help with positioning, monitor for distress.
- After: monitor for complications and teach warning signs to report — vaginal fluid leakage, bleeding, cramping, fever, or decreased fetal movement. Results take time (cell culture); coordinate result communication and follow-up counseling.
Glucose screening for gestational diabetes (~24–28 weeks)
Around 24–28 weeks, most pregnant people are screened for gestational diabetes (GDM). The most common approach is two-step: a glucose challenge test (a glucose drink followed by a blood draw) as the screen, and — if abnormal — a longer oral glucose tolerance test as the diagnostic confirmation. Some settings use a one-step diagnostic approach; protocols vary. The nurse explains fasting/meal instructions, coordinates the test, and educates about follow-up. GDM usually resolves after birth but is a marker for future type 2 diabetes (see Chapter 12); no numeric cutoffs are memorized here — thresholds follow current guidelines and institutional policy.
Integrated and sequential screening
First- and second-trimester results are often combined into a single risk estimate:
- Sequential screening — first-trimester results are reported, and second-trimester markers refine the estimate.
- Integrated screening — all markers from both trimesters are analyzed together and reported as one result.
This is why the two testing topics belong together: the second trimester does not "replace" the first — it completes it.
The nursing role and scope
Educate before and after testing; support informed, non-directive decisions; coordinate scheduling, labs, and referrals (genetics, maternal-fetal medicine); document and communicate results per facility policy; monitor post-procedure; teach warning signs. Diagnosing, interpreting results, and managing findings are provider responsibilities — the nurse explains, supports, and connects. Testing menus and timing vary by institution and jurisdiction; practice within local policy and current guidelines.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Quad screen | Anatomy ultrasound | Quad screen is a blood test estimating risk (chromosomal + neural tube); the anatomy scan is the structural ultrasound — complementary, not interchangeable |
| Elevated AFP = the baby has spina bifida | Elevated AFP = risk marker needing follow-up | Dating errors, twins, and other conditions can raise AFP; follow-up, not diagnosis, comes first |
| Amniocentesis | CVS | Amniocentesis samples amniotic fluid in the second trimester (after ~15 weeks); CVS samples placental tissue in the first (~10–13 weeks) |
| Abnormal glucose screen = gestational diabetes | Screen needs confirmation | A glucose challenge test is screening; an abnormal screen is followed by the diagnostic oral glucose tolerance test per protocol |
| Second-trimester tests replace first-trimester tests | They combine (integrated/sequential) | Both trimesters' markers merge into the most accurate single risk estimate |
| GDM found in the second trimester = preexisting diabetes | GDM = glucose intolerance first recognized in pregnancy | GDM arises from pregnancy's insulin resistance and usually resolves after birth (see Chapter 12) |
| "Normal anatomy scan" = perfectly healthy baby | "No problems found on this exam" | Scans have limits; some conditions are not detectable prenatally |
Eli explains
The same idea, in plain words
Explain it like I’m 10
In the middle months, the baby gets a big "checkup." The doctor takes a very detailed look at all the baby's body parts with a special camera (the anatomy ultrasound). A blood test from the parent gives a "risk estimate" — a rough idea, not a yes-or-no — for a few conditions. If the estimates look concerning, or if the family wants a definite answer, a doctor can take a tiny sample of the water around the baby with a thin needle (amniocentesis) and check the baby's cells directly. There's also a sugary-drink test to check how the parent's body handles sugar. Each test has its own job, and the nurse explains each one before it happens.
Worked example
At 18 weeks, a pregnant person's quad screen comes back with an elevated AFP level. The nurse's handling of the result models best practice:
- Put the result in context: "An elevated AFP is a screening finding, not a diagnosis. It means we should look more closely — and several reasons can explain it, including miscalculated dates or twins."
- Verify the basics: the nurse checks the dating — was the ultrasound dating consistent with the last menstrual period estimate? A dating error is a common explanation for an out-of-range AFP.
- Offer the next steps: a detailed ultrasound of the fetal spine and skull, genetic counseling, and the option of amniocentesis to measure amniotic fluid AFP for a more definitive answer.
- Support and coordinate: schedule the follow-up, answer questions honestly, and stay present while the family processes the uncertainty — making clear the decision to pursue or decline further testing is theirs.
Why this works: the nurse neither dismissed the result nor treated it as a diagnosis. Elevated AFP → systematic follow-up (dating check → detailed ultrasound → possible amniocentesis) is exactly the pathway, and the education kept the family informed and in control.
Key takeaways
- Anatomy ultrasound ~18–22 weeks — the detailed structural survey; also checks placental location and amniotic fluid.
- Quad screen ~15–22 weeks = AFP + hCG + estriol + inhibin A — the only common blood screen covering open neural tube defects (via AFP).
- Elevated AFP → neural tube defect risk — but also consider dating errors and multiple gestation; follow-up is a detailed ultrasound and possibly amniocentesis.
- Amniocentesis = the second-trimester diagnostic test (amniotic fluid, fetal cells, typically after ~15 weeks); CVS was the first-trimester option.
- Post-amniocentesis teaching: report vaginal fluid leakage, bleeding, cramping, fever, or decreased fetal movement.
- Rh-negative persons receive anti-D immune globulin per protocol after invasive procedures like amniocentesis.
- GDM screening ~24–28 weeks; abnormal screen → diagnostic glucose tolerance test per local protocol.
- First- and second-trimester results combine (sequential/integrated screening) for the most accurate risk estimates.
- Screening estimates risk; only diagnostic tests give definitive answers. Interpretation is a provider role; nurses educate, support, coordinate, and monitor — within local policy.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What four markers make up the quad screen, and which one covers neural tube defects?
Show answer
AFP, hCG, estriol, and inhibin A. AFP (alpha-fetoprotein) is the marker used to screen for open neural tube defects; the other three contribute to the chromosomal risk estimate.
A pregnant person asks why the anatomy scan is "just" at 18–22 weeks. What should the nurse explain?
Show answer
The scan is timed so fetal structures are large enough to be seen clearly while the pregnancy is still early enough for decisions; some structures (e.g., heart chambers) are best evaluated in this window. It is the standard timing for the anatomy survey.
What are the key post-amniocentesis warning signs to teach?
Show answer
Vaginal fluid leakage, bleeding, cramping, fever, and decreased fetal movement — any should be reported promptly.
Why must gestational age be accurate when interpreting an AFP result?
Show answer
Because AFP levels change with gestational age; if the dates are wrong, the AFP value can look falsely elevated or low, causing unnecessary worry or a missed signal. Dating must be verified before interpreting the result.
How do first- and second-trimester screening results work together?
Show answer
First- and second-trimester markers are combined in sequential or integrated screening to produce a single, more accurate risk estimate — the second trimester completes, rather than replaces, the first.
Why does an Rh-negative person receive anti-D immune globulin after amniocentesis, per protocol?
Show answer
Anti-D immune globulin prevents an Rh-negative person from developing antibodies to Rh-positive fetal blood cells that could be introduced during the invasive procedure, protecting future pregnancies. Administration follows protocol.
Study toolsKey vocabulary
Key vocabulary
- Anatomy ultrasound
- Detailed ~18–22 week scan surveying fetal structures, placenta, and fluid
- Quad screen
- Second-trimester blood screen measuring AFP, hCG, estriol, inhibin A
- AFP (alpha-fetoprotein)
- Fetal liver protein measurable in maternal blood
- Neural tube defect
- A fetal spine/brain development problem (e.g., spina bifida, anencephaly)
- Amniocentesis
- Diagnostic sampling of amniotic fluid (fetal cells) after ~15 weeks
- Glucose challenge / tolerance test
- Screen, then diagnostic confirmation, for gestational diabetes
- Integrated / sequential screening
- Combining first- and second-trimester markers into one refined risk estimate
- Karyotype / chromosomal microarray
- Laboratory analyses of fetal cells from CVS or amniocentesis
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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