Diagnosis of Pregnancy and Dating MCQ — NEET PG Practice Question | NEETPGAI
Diagnosis of Pregnancy and Dating
medium
baby OBG
A 28-year-old woman attends the antenatal clinic at 12 weeks gestation. She has regular 28-day cycles and reports her last menstrual period (LMP) as 12 weeks ago. Transvaginal ultrasound shows a crown-rump length (CRL) of 58 mm. Which feature best distinguishes dating by LMP from dating by first-trimester ultrasound in this scenario?
A. LMP dating is more reliable in women with polycystic ovary syndrome; ultrasound CRL requires normal ovulation
B. LMP dating requires knowledge of cycle regularity; ultrasound CRL can be performed at any gestational age
C. LMP dating is accurate to ±3 days; ultrasound CRL is accurate to ±5 days
D. LMP dating assumes a 28-day cycle; ultrasound CRL is independent of menstrual cycle length
Explanation
Comparison: LMP Dating vs First-Trimester Ultrasound Dating
Key Discriminating Feature
Key Point
LMP dating is inherently dependent on the assumption of a standard 28-day menstrual cycle, whereas ultrasound dating by crown-rump length (CRL) in the first trimester is a direct biometric measurement that is independent of cycle length.
Why This Matters Clinically
Clinical Pearl
In women with irregular cycles, anovulation, or those taking hormonal contraceptives before conception, LMP dating can be significantly inaccurate. Ultrasound CRL bypasses this limitation entirely by measuring the actual fetal size.
Accuracy Comparison
Table
Dating Method
Accuracy Window
Dependency
Best Timing
LMP (regular cycles)
±3–5 days
Assumes 28-day cycle
Retrospective only
First-trimester CRL
±3–5 days (11–14 weeks)
Fetal biometry only
11–14 weeks gestation
Second-trimester BPD/AC
±2 weeks
Fetal biometry
15–20 weeks
Third-trimester biometry
±3–4 weeks
Fetal biometry
>20 weeks
High-YieldNEET PG
When LMP and ultrasound dating disagree by >5 days in the first trimester, ultrasound takes precedence because it is not confounded by cycle length or ovulation timing.
Why LMP Dating Fails in Irregular Cycles
1.
Ovulation may occur on day 14 (standard assumption) or day 10–21 (irregular cycles)
2.
Conception date is unpredictable relative to LMP
3.
Ultrasound directly measures fetal size, eliminating this uncertainty
Mnemonic
CYCLE — Cycles vary; Ultrasound Confirms; LMP Estimates; Early scan best; Dating shifts with discrepancy.
Williams Obstetrics 25e Ch 4
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