## Obstetric Ultrasound Dating in the Second Trimester ### CRL Measurement and Gestational Age Calculation **Key Point:** Crown-rump length (CRL) is the gold standard for gestational age assessment in the first and early second trimester (up to 13 weeks + 6 days). At 13–14 weeks, CRL measurement transitions to less reliable accuracy. A CRL of 68 mm corresponds to approximately **13 weeks of gestation** using standard biometric tables [cite:Hadlock 1984 / ISUOG Guidelines]. ### Accuracy of Dating by Trimester | Trimester | Measurement | Accuracy (±) | Optimal Timing | |-----------|-------------|--------------|----------------| | First | CRL | ±3–5 days | 8–13 weeks | | Early Second | CRL | ±5–7 days | 13–20 weeks | | Late Second | Biparietal diameter (BPD), femur length (FL) | ±10–14 days | 20–24 weeks | | Third | BPD, FL, abdominal circumference (AC) | ±3–4 weeks | >24 weeks | **High-Yield:** At **13 weeks gestation**, CRL measurement retains its highest accuracy with a margin of error of **±3 days**. This is the last window where CRL provides optimal precision. ### Clinical Pearl When menstrual history is unreliable (as in this case), first-trimester or early second-trimester ultrasound becomes the most reliable method for establishing gestational age. A CRL of 68 mm at 13 weeks ± 3 days is the most accurate dating statement for this patient. ### Why This Matters **Key Point:** Accurate dating in the first and early second trimester is critical for: - Correct identification of preterm delivery risk - Appropriate timing of aneuploidy screening (nuchal translucency at 11–13 weeks + 6 days) - Accurate assessment of fetal growth in later scans - Determination of delivery timing in high-risk pregnancies [cite:ISUOG Guidelines on Ultrasound Screening for Fetal Anomalies] 
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