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AN81.1-3 | Prenatal Diagnosis — Practice Quiz
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A 37-year-old woman undergoes NIPT (non-invasive prenatal testing) at 11 weeks and receives a high-risk result for trisomy 21. NIPT uses cell-free fetal DNA obtained from:
Correct! NIPT analyses CELL-FREE FETAL DNA (cffDNA) from PLACENTAL TROPHOBLAST CELLS that circulate in the MATERNAL BLOOD (plasma) from about 10 weeks. No procedure required on the fetus or amniotic cavity — only a maternal blood sample.
NIPT: maternal blood → cffDNA (from syncytiotrophoblast apoptosis) → next-generation sequencing → trisomy 21, 18, 13, sex chromosome aneuploidies. Sensitivity >99% for T21. BUT: SCREENING ONLY; false positives occur (confined placental mosaicism, maternal malignancy). Must confirm with amniocentesis/CVS if positive.
Incorrect. NIPT uses cffDNA from MATERNAL BLOOD. It is a SCREENING test (not diagnostic). A positive NIPT result must always be confirmed by invasive testing (amniocentesis or CVS) with karyotyping.
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The routine fetal anomaly scan is performed to detect structural congenital abnormalities (cardiac defects, NTDs, renal anomalies). What is the optimal gestational age window for this scan?
Correct! The ANOMALY SCAN (also called the "morphology scan" or "level 2 scan") is performed at 18–20 WEEKS. At this stage: organs are well formed and large enough to visualise on ultrasound; amniotic fluid provides an acoustic window; fetal size allows detailed anatomy. The 11–13+6 scan is the NUCHAL TRANSLUCENCY scan (1st trimester screening).
Two main scans: (1) NT scan at 11–13+6 wks: dating + trisomy screening (NT measurement, nasal bone). (2) Anomaly scan at 18–20 wks: detailed anatomy survey for structural defects. PCPNDT Act: sex must not be disclosed at either scan.
Incorrect. The anomaly scan = 18–20 weeks. The 11–13+6 scan = nuchal translucency + dating. The anomaly scan at 18–20 wks detects: cardiac defects, NTDs, renal anomalies, skeletal dysplasias, abdominal wall defects, facial clefts.
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A maternal serum triple screen at 16 weeks shows: AFP (↓ low), hCG (↑ high), uE3 (↓ low). This pattern is most consistent with:
Correct! TRISOMY 21 (Down syndrome) pattern: AFP ↓ + hCG ↑ + uE3 ↓ (mnemonic: "AhA" — AFP down, hCG UP, uE3 down). In contrast: OPEN NTD = AFP ↑↑ (leaks from open neural tube). Trisomy 18 = ALL THREE markers low.
Triple screen markers for T21: AFP↓ hCG↑ uE3↓. Quadruple adds inhibin A ↑. For open NTD: AFP↑↑ (amniotic fluid AFP also ↑). AFP is produced by fetal yolk sac and liver; open NTDs allow AFP to leak into amniotic fluid and cross to maternal serum.
Incorrect. AFP ↓ + hCG ↑ + uE3 ↓ = DOWN SYNDROME (T21). Remember: Open NTD = AFP ↑↑ (leaks through open defect). T18 = ALL down (AFP↓ + hCG↓ + uE3↓). T21 = AFP↓ + hCG↑ + uE3↓.
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The optimal gestational age for amniocentesis for chromosomal analysis is:
Correct! AMNIOCENTESIS is optimally performed at 15–20 WEEKS. Before 15 weeks: higher miscarriage risk + associated with fetal club foot (early amniocentesis). After 20 weeks: results come late (culture takes 2 weeks → result at 22+ weeks → termination has higher morbidity).
Amniocentesis: 15–20 wks; 15–20 mL fluid; needle avoids placenta under USG guidance; cells cultured 10–14 days for karyotype. Miscarriage risk 0.1–0.5% (1 in 200–1000). Alternative: FISH for rapid result in 48–72 hrs (for common aneuploidies only).
Incorrect. Amniocentesis optimal window = 15–20 weeks. Early amniocentesis (<15 weeks) = higher risk of miscarriage and fetal club foot. CVS (10–13 weeks) allows earlier diagnosis.
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A 38-year-old primigravida at 16 weeks has a positive NIPT result for trisomy 21. She undergoes amniocentesis. The amniocentesis result returns as 46,XX (normal female karyotype). The most likely explanation is:
Correct! This scenario illustrates CONFINED PLACENTAL MOSAICISM (CPM) — a trisomy present in the TROPHOBLAST cells (detected by NIPT, which reads cffDNA from trophoblast) but NOT in the fetus itself (amniocentesis samples fetal cells, which are normal 46XX). The trisomy occurred as a mitotic error in trophoblast and was "corrected" in the embryo.
NIPT limitation: reads TROPHOBLAST DNA (placenta), not fetal DNA. If trisomy is confined to placenta (CPM), NIPT is positive but fetus is normal (amniocentesis normal). This explains why positive NIPT must always be confirmed by invasive testing. CPM occurs in ~1–2% of pregnancies.
Incorrect. This is CONFINED PLACENTAL MOSAICISM — NIPT reads trophoblast DNA (placenta has trisomy) but fetal cells are normal (amniocentesis shows true fetal karyotype 46XX). This is the most important limitation of NIPT — it reads placental, not fetal, DNA.
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The procedure-related pregnancy loss rate (miscarriage risk) associated with mid-trimester amniocentesis (15–20 weeks) is approximately:
Correct! Amniocentesis miscarriage risk = approximately 0.1–0.5% (1 in 200 to 1 in 1,000). This is the background quoted risk that must be discussed in pre-procedure counselling. This risk must be weighed against the risk of the condition being tested (e.g., 1-in-100 risk of trisomy 21).
Miscarriage risks: Amniocentesis = 0.1–0.5%. CVS = 1–2% (higher because earlier, more vascular implantation site, and the techniques are technically more demanding). NIPT = 0% (no fetal intervention). Counselling must include these figures before obtaining consent.
Incorrect. Amniocentesis = 0.1–0.5% miscarriage risk. CVS = higher at 1–2%. These figures are critical for counselling. The risk is lower with experienced operators (in skilled hands: ~0.1%).
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The primary clinical advantage of chorionic villus sampling (CVS) over amniocentesis is:
Correct! The PRIMARY ADVANTAGE of CVS is TIMING — it can be performed at 10–13 weeks (1st trimester), giving results much earlier than amniocentesis (2nd trimester). If the fetus is found to have a lethal/serious anomaly, 1st trimester medical termination (safer, less traumatic) is possible vs a 2nd trimester termination after amniocentesis.
CVS vs amniocentesis: CVS = earlier (10–13 wks), faster result, 1st trimester termination option. Amniocentesis = lower miscarriage risk, can measure AFP (NTDs), less CPM issue. Both provide chromosomal karyotype. Choose CVS for early diagnosis in high-risk cases; amniocentesis for 2nd trimester indications.
Incorrect. CVS ADVANTAGES: earlier (1st trimester), rapid result (48–72 hrs direct prep vs 2 weeks). CVS DISADVANTAGES: higher miscarriage risk (1–2% vs 0.1–0.5%), cannot measure AFP (no fluid), and IS affected by CPM (false positives). AFP cannot be assessed by CVS.
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CVS performed before 10 weeks of gestation is associated with which specific fetal complication not seen with amniocentesis?
Correct! CVS performed BEFORE 10 WEEKS is associated with TRANSVERSE LIMB REDUCTION DEFECTS — absence or shortening of hands, fingers, or limbs (most commonly). The mechanism is vascular disruption or direct compression during a critical period of limb development. After 10 weeks, this risk is minimal (1 in 3,000).
CVS before 10 weeks → transverse limb reduction defects (oromandibular-limb hypogenesis spectrum). Current guidelines: CVS not before 10+0 weeks. After 10 weeks: risk ~1 in 3,000 (acceptable given the information gained). This is a classic examination question differentiating CVS from amniocentesis complications.
Incorrect. Limb reduction defects are the specific complication of CVS before 10 weeks. After 10 weeks the risk is very low (1 in 3,000). This is why CVS is NOT performed before 10 weeks. Amniocentesis is not associated with limb defects.
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CVS can be performed via two routes. For a posteriorly implanted placenta where transcervical access is straightforward, which route is preferred?
Correct! For POSTERIOR placentation where the cervix provides direct access to the placenta, the TRANSCERVICAL ROUTE (flexible plastic catheter through the cervix under USG guidance) is preferred. For ANTERIOR or FUNDAL placentas, the TRANSABDOMINAL route (needle through anterior abdominal wall) is used.
CVS route = based on placental position: posterior → transcervical; anterior/fundal → transabdominal. Both give the same genetic material. Transabdominal CVS is preferred in most Indian centres regardless of position (operator preference). Transcervical CVS: more risk of infection/bleeding.
Incorrect. CVS route selection: TRANSCERVICAL = posterior implantation (easy cervical access). TRANSABDOMINAL = anterior or fundal implantation (needle through abdominal wall). Both routes sample the same tissue (chorionic villi) and give equivalent results.
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A couple requests CVS at 11 weeks for chromosomal analysis. After the procedure, the husband directly asks the sonologist: "Is it a boy or a girl?" Under the PCPNDT Act 1994 (India), the correct response is:
Correct! The PCPNDT Act 1994 PROHIBITS disclosure of fetal sex determined by ANY prenatal diagnostic technique (USG, amniocentesis, CVS, NIPT) — regardless of the stated reason, consent forms, or the parent's identity. Violation = criminal offence: 3–5 years imprisonment and cancellation of registration.
PCPNDT Act 1994: prohibits sex selection + sex disclosure from PND. Penalty = 3–5 years prison + fine. Exception: karyotype reveals sex chromosome anomaly (Turner, Klinefelter, etc.) — sex is part of the medical diagnosis, not sex selection. All PND centres must be registered; mandatory forms and records.
Incorrect. PCPNDT Act = ABSOLUTE PROHIBITION on sex disclosure. No exceptions for consent forms, mother vs father, or medical indications. The only exception is when the karyotype reveals a SEX CHROMOSOME ANOMALY (e.g., Turner syndrome 45X, Klinefelter 47XXY) — where sex is part of the diagnosis, not sex selection.
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