A 28-year-old primigravida presents to the antenatal clinic at 12 weeks of gestation for dating of pregnancy. Transvaginal ultrasound is performed. Which of the following is the most common and most reliable parameter for accurate dating of pregnancy in the first trimester?

Explanation

## Accuracy of First Trimester Dating Parameters **Key Point:** Crown-rump length (CRL) is the most accurate parameter for dating pregnancy in the first trimester, with an accuracy of ±3–5 days when performed between 8–14 weeks of gestation. ### Comparative Accuracy by Trimester | Parameter | Timing | Accuracy | Notes | |-----------|--------|----------|-------| | Gestational sac diameter | 8–13 weeks | ±3–5 mm | Early marker, less precise than CRL | | **Crown-rump length** | **8–14 weeks** | **±3–5 days** | **Gold standard for first trimester** | | Biparietal diameter | 14–20 weeks | ±1–2 weeks | Better in second trimester | | Femur length | 14–20 weeks | ±1–2 weeks | Better in second trimester | **High-Yield:** The accuracy of ultrasound dating decreases as pregnancy advances: - First trimester: ±3–5 days - Second trimester: ±1–2 weeks - Third trimester: ±3–4 weeks ### Why CRL is Superior in First Trimester 1. Minimal biological variation between fetuses in early pregnancy 2. Least affected by genetic and nutritional factors 3. Measured from the top of the fetal head to the tip of the buttocks 4. Performed transvaginally for optimal visualization **Clinical Pearl:** Gestational sac diameter is useful for confirming intrauterine pregnancy and detecting early pregnancy loss, but CRL is the preferred dating tool once the embryo is measurable (8 weeks onwards). **Mnemonic:** **CRL-FIRST** — Crown-Rump Length is First Trimester's best dating tool.