Diagnosis of Pregnancy and Dating MCQ — NEET PG Practice Question | NEETPGAI
Diagnosis of Pregnancy and Dating
medium
baby OBG
A 32-year-old primigravida at 14 weeks gestation by menstrual history presents for dating ultrasound. Transvaginal ultrasound shows a crown-rump length of 68 mm. Which is the most appropriate next investigation to refine dating accuracy if there is a discrepancy between LMP and ultrasound measurement?
A. Repeat transvaginal ultrasound in 2 weeks
B. Transabdominal ultrasound with biometry (biparietal diameter, femur length)
C. Maternal serum alpha-fetoprotein (AFP)
D. Serum progesterone level
Explanation
Refining Pregnancy Dating When Discrepancy Exists Between LMP and Early Ultrasound
Key Point
When there is a discrepancy between menstrual dating and first-trimester ultrasound, the most appropriate next step depends on gestational age and the magnitude of discrepancy. At 14 weeks, transabdominal ultrasound with biometry (biparietal diameter, femur length) is the next best investigation.
Understanding Dating Discrepancies
High-YieldNEET PG
A CRL of 68 mm at 14 weeks corresponds to approximately 13 weeks + 2–3 days (±3–5 days accuracy). A discrepancy of ≥2 weeks between LMP and first-trimester ultrasound warrants investigation.
Why Transabdominal Ultrasound with Biometry?
1.
Transition to second-trimester biometry: At 14 weeks, biparietal diameter (BPD) and femur length (FL) become measurable and reliable
2.
Better accuracy in second trimester: BPD and FL have ±1–2 weeks accuracy in the second trimester (14–20 weeks)
3.
Complements CRL: Provides independent confirmation of gestational age
4.
Assesses fetal anatomy: Allows evaluation for structural anomalies (important at 18–20 weeks)
5.
Non-invasive, safe: Standard antenatal care investigation
Accuracy of Dating Methods by Gestational Age
Table
Gestational Age
Best Method
Accuracy
Biometric Parameter
≤13+6 weeks
Transvaginal ultrasound
±3–5 days
CRL
14–20 weeks
Transabdominal ultrasound
±1–2 weeks
BPD, FL, HC
21–30 weeks
Transabdominal ultrasound
±2–3 weeks
BPD, FL, AC
>30 weeks
Transabdominal ultrasound
±3–4 weeks
FL, AC, EFW
Clinical Pearl
Once dating is established in the first or early second trimester, it should NOT be changed by later ultrasounds. If a discrepancy exists, the earliest reliable ultrasound dating should be used.
Mnemonic
CAFE — CRL (first trimester), Abdomen (third trimester), Femur (second trimester), Early dating is most accurate.
Williams Obstetrics 26e Ch 5
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