Correct Answer: A. 11-14 weeks
Nuchal translucency (NT) measurement is a first-trimester ultrasound marker for chromosomal abnormalities, particularly trisomy 21. The ideal window for NT evaluation is 11–14 weeks of gestation, with the crown-rump length (CRL) between 45–84 mm. This timing is critical because the subcutaneous fluid collection at the nape of the neck is most reliably visualized during this narrow gestational window. Before 11 weeks, the NT is too small to measure accurately; after 14 weeks, the fluid resorbs physiologically, making measurement impossible or unreliable. According to Indian guidelines and international consensus (ISUOG, AIUM), NT screening combined with maternal serum markers (PAPP-A and β-hCG) at this gestational age provides a detection rate of 80–90% for trisomy 21 with a false-positive rate of 5%. This combined first-trimester screening is the standard of care in Indian tertiary centers and is recommended by the Indian College of Obstetricians and Gynaecologists (ICOG) for all singleton pregnancies. The measurement must be taken in a mid-sagittal plane with the fetus in a neutral position, and the calipers should be placed from the inner border of the nuchal fold to the inner border of the skin line.
Why the other options are wrong
B. 14-16 weeks — This is wrong because by 14–16 weeks, the nuchal translucency has already begun to resorb physiologically. While a measurement may still be possible at 14 weeks (the upper limit of the ideal window), by 16 weeks the fluid collection is largely resolved, making accurate measurement unreliable. This timing falls into the second trimester and misses the first-trimester screening window. NBE may trap students who confuse this with the timing for second-trimester markers like nasal bone assessment (18–24 weeks). C. 6-8 weeks — This is wrong because at 6–8 weeks of gestation, the nuchal translucency is too small to be visualized or measured reliably on ultrasound. The CRL at this gestational age is <45 mm, which is below the threshold for accurate NT assessment. While early dating scans are performed at this stage, NT screening is not performed. NBE may trap students who confuse early pregnancy dating scans with NT screening, which are different objectives. D. 16-20 weeks — This is wrong because by 16–20 weeks, the nuchal translucency has largely resorbed and is no longer a reliable first-trimester marker. This timing is in the second trimester, when other structural markers (nasal bone, cardiac defects, echogenic bowel) are assessed instead. The window for NT measurement has closed. NBE may trap students who confuse second-trimester anatomy scan timing with first-trimester screening markers.
High-Yield Facts
- 11–14 weeks gestation (CRL 45–84 mm) is the only window for accurate nuchal translucency measurement; before 11 weeks it is too small, after 14 weeks it resorbs.
- NT ≥3 mm is considered abnormal and increases risk of trisomy 21; combined with maternal serum markers, detection rate is 80–90%.
- First-trimester combined screening (NT + PAPP-A + β-hCG) is the standard of care in Indian tertiary centers per ICOG guidelines.
- Mid-sagittal plane with neutral fetal position is mandatory for accurate NT measurement; incorrect plane leads to false-positive results.
- NT resorbs physiologically after 14 weeks; second-trimester markers (nasal bone, cardiac defects) replace NT for screening after this window.
Mnemonics
NT Window: 11-14 11 weeks = earliest reliable measurement (CRL ≥45 mm). 14 weeks = latest before resorption. Think: "11 to 14, don't be late—after 14 the fluid's gone, mate!" CRL Cutoff for NT 45–84 mm is the CRL range for NT measurement. Below 45 mm = too early; above 84 mm = too late. Memory: "45 to 84, that's the gate."
NBE Trap
NBE pairs "nuchal translucency" with "second-trimester anatomy scan" to lure students who confuse first-trimester screening markers (NT) with second-trimester structural assessment (nasal bone, cardiac defects). The trap is the gestational age range—students may select 14–16 or 16–20 weeks thinking of the anatomy scan window.
Clinical Pearl
In Indian practice, first-trimester combined screening (NT + serum markers) is increasingly offered in metropolitan centers as a non-invasive alternative to invasive testing (amniocentesis). A thickened NT (≥3 mm) in a 32-year-old primigravida warrants detailed anatomy scan and consideration of cell-free fetal DNA testing—this combined approach reduces unnecessary invasive procedures and improves maternal acceptance in Indian populations.
_Reference: DC Dutta's Textbook of Obstetrics, Ch. 10 (Antenatal Care & Screening); ICOG Guidelines on Antenatal Screening for Chromosomal Abnormalities_