## Accuracy of Dating Methods in First Trimester **Key Point:** First trimester dating by ultrasound is the most accurate method, with an error margin of ±3–5 days. Clinical examination of uterine size is subjective and unreliable for precise dating. ### Reliable First Trimester Dating Methods | Method | Accuracy | Timing | Notes | |--------|----------|--------|-------| | Transvaginal ultrasound (gestational sac) | ±3–5 days | 8–13 weeks | Most accurate in early pregnancy | | Transabdominal ultrasound (CRL) | ±3–5 days | 11–14 weeks | Gold standard for first trimester | | Serum β-hCG + ultrasound | ±3–5 days | 4–8 weeks | Useful when ultrasound dating uncertain | | Clinical uterine size | ±4 weeks | Any trimester | Highly subjective; affected by parity, BMI, fibroids | **High-Yield:** Uterine size on bimanual examination is influenced by multiple factors (parity, uterine fibroids, obesity, bladder fullness) and cannot reliably date pregnancy. It is useful only for confirming pregnancy is present, not for precise gestational age. **Clinical Pearl:** A 12-week uterus is approximately at the level of the symphysis pubis on abdominal examination, but this is a rough landmark only. Ultrasound should always be used for accurate dating in the first trimester. ### Why Other Methods Are Reliable - **Transvaginal ultrasound:** Visualizes gestational sac as early as 4.5–5 weeks; mean sac diameter correlates precisely with gestational age. - **Crown-rump length (CRL):** Measured from 11–14 weeks; accuracy ±3–5 days; becomes less accurate after 14 weeks. - **β-hCG:** Doubles every 48–72 hours in early pregnancy; combined with ultrasound, refines dating when LMP uncertain. **Warning:** Do not rely on clinical examination alone for dating. Always confirm with ultrasound in the first trimester.
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