## Accuracy of Biometric Parameters for Pregnancy Dating **Key Point:** Head circumference (HC) is the most accurate single biometric parameter for dating in the second trimester, with an accuracy margin of ±2 weeks. ### Comparison of Biometric Accuracy by Trimester | Parameter | 1st Trimester | 2nd Trimester | 3rd Trimester | |---|---|---|---| | Crown-rump length (CRL) | ±3–5 days | Not used | Not used | | Biparietal diameter (BPD) | — | ±2–3 weeks | ±3–4 weeks | | Head circumference (HC) | — | ±2 weeks | ±3–4 weeks | | Femur length (FL) | — | ±3–4 weeks | ±4–5 weeks | | Abdominal circumference (AC) | — | ±4–5 weeks | ±5–6 weeks | **High-Yield:** The accuracy of all biometric parameters **decreases with advancing gestational age**. CRL in the first trimester is the gold standard (±3–5 days), but in the second trimester, HC becomes the most accurate single parameter. ### Clinical Application **Clinical Pearl:** When multiple parameters are available: - Use the **average of 2–3 parameters** for better accuracy - HC and BPD are preferred over FL and AC in the second trimester - AC is the least reliable for dating and is better used for growth assessment **Mnemonic:** **CHFA** — **C**RL (1st trim), **H**C (2nd trim), **F**L (3rd trim), **A**C (growth, not dating) **Tip:** In clinical practice, if dating by ultrasound differs from LMP by >2 weeks in the second trimester, the ultrasound date is preferred.
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