## Second Trimester Dating: Biometric Accuracy **Key Point:** **Head circumference (HC)** is the most reliable single biometric parameter for dating pregnancies in the second trimester (13–20 weeks), with an accuracy of ±3–5 days. ### Comparison of Second Trimester Biometric Parameters | Parameter | Accuracy (±days) | Notes | | --- | --- | --- | | **Head Circumference (HC)** | **3–5 days** | **Most reliable; unaffected by head shape variations (dolichocephaly/brachycephaly)** | | Biparietal Diameter (BPD) | 4–7 days | Good but affected by head shape (cephalic index); less reliable than HC | | Femur Length (FL) | 4–6 days | Reliable but slightly more variable due to ethnic/genetic differences in long bone length | | Abdominal Circumference (AC) | 10–20 days | Least reliable; highly affected by fetal nutrition, growth, and liver size | ### Why HC is Superior in the Second Trimester 1. **Shape-independent:** Unlike BPD, HC is not affected by head shape variations such as dolichocephaly or brachycephaly, making it more consistent across fetuses. 2. **Less nutritional influence:** HC is not affected by fetal nutritional status, unlike AC. 3. **Less ethnic variation:** HC shows less inter-ethnic variability compared to FL, which can differ significantly across populations (e.g., Asian vs. Caucasian femur length norms). 4. **Consistent growth trajectory:** HC grows predictably throughout the second trimester and is the preferred single parameter per standard obstetric ultrasound references (Callen, Rumack). ### Hierarchy of Dating Parameters by Trimester - **First trimester (up to 13 weeks):** Crown-rump length (CRL) — gold standard (accuracy ±3–5 days) - **Second trimester (13–20 weeks):** Head circumference (HC) — most reliable single parameter - **Third trimester (>28 weeks):** All parameters less reliable (±2–3 weeks); composite biometry preferred **High-Yield:** BPD is commonly tested but is inferior to HC because it is affected by the cephalic index. HC corrects for this by measuring the entire circumference of the skull, making it the preferred single dating parameter in the second trimester. **Clinical Pearl:** In clinical practice, a composite of BPD + HC + FL + AC is used for improved accuracy. However, when a single parameter is required for second trimester dating, **HC is the most reliable** per Callen's Ultrasound in Obstetrics and Gynecology and Rumack's Diagnostic Ultrasound. **Mnemonic:** **"HC = Head Count on reliability"** — Head Circumference is the most reliable single parameter in the second trimester. [cite: Callen's Ultrasound in Obstetrics and Gynecology 6e, Ch 4; Rumack CM, Diagnostic Ultrasound 4e]
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