A 32-year-old woman from Mumbai attends the antenatal clinic at 12 weeks of gestation (by LMP). On transvaginal ultrasound, the crown-rump length (CRL) measures 52 mm. The nuchal translucency (NT) is 1.8 mm. Fetal cardiac rate is 155 bpm. Based on the CRL measurement, what is the revised gestational age, and what is the margin of error for dating at this stage?
A. 13 weeks 2 days ± 5 days
B. 12 weeks 0 days ± 7 days
C. 11 weeks 5 days ± 3 days
D. 12 weeks 3 days ± 3 days
Explanation
Crown-Rump Length (CRL) for Dating in the First Trimester
CRL and Gestational Age Correlation
Key Point
The crown-rump length (CRL) is the most accurate biometric parameter for dating pregnancy in the first trimester, with an accuracy of ±3 days when performed between 8–14 weeks of gestation. CRL grows at approximately 1.3 mm per day in the first trimester.
CRL to Gestational Age Conversion Table
Table
CRL (mm)
Gestational Age (weeks)
Accuracy
15–20
8–9
±3 days
21–30
9–10
±3 days
31–40
10–11
±3 days
41–50
11–12
±3 days
51–60
12–13
±3 days
61–70
13–14
±3 days
High-YieldNEET PG
A CRL of 52 mm corresponds to approximately 12 weeks 3 days of gestation with a margin of error of ±3 days. This is the gold standard for first-trimester dating.
Clinical Application
Clinical Pearl
In this case, the patient's LMP-based age is 12 weeks, but the ultrasound CRL suggests 12 weeks 3 days—a difference of only 3 days, which is within the expected accuracy range. The revised age would be 12 weeks 3 days, and this should be used for all subsequent pregnancy management and delivery date estimation.
Why CRL is Superior to Other First-Trimester Markers
1.
Gestational sac diameter: Accurate to ±3–5 days (4–8 weeks).
2.
Yolk sac diameter: Limited utility; used mainly to confirm intrauterine pregnancy.
3.
CRL: Accurate to ±3 days (8–14 weeks) — most reliable.
4.
Biparietal diameter (BPD): Accurate to ±1–2 weeks (15–20 weeks); less accurate than CRL.
5.
Femur length: Accurate to ±2–3 weeks (15–20 weeks); less accurate than CRL.
Mnemonic
CRL-GOLD — Crown-Rump Length is the GOLD standard for first-trimester dating (±3 days).
Nuchal Translucency (NT) Note
Warning
NT (1.8 mm in this case) is used for aneuploidy risk assessment, not dating. An NT of <2.5 mm is considered normal at 11–14 weeks. Do not confuse NT measurement with dating parameters.
Fetal Cardiac Rate
The fetal heart rate of 155 bpm is normal for 12 weeks of gestation (normal range: 120–160 bpm). This is a reassuring sign but is not used for dating.
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