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?

## 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-Yield:** 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 | 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** — **C**RL (first trimester), **A**bdomen (third trimester), **F**emur (second trimester), **E**arly dating is most accurate. [cite:Williams Obstetrics 26e Ch 5]