## Fetal Cardiac Activity on Transvaginal USG **Key Point:** Fetal cardiac activity (FCA) is the most reliable sign of a viable intrauterine pregnancy and is the gold standard for confirming viability. ### Timeline of Cardiac Visibility | Gestational Age | Imaging Modality | Finding | | --- | --- | --- | | 5–6 weeks | Transvaginal USG | Fetal heart rate becomes visible; mean sac diameter (MSD) ~15–20 mm | | 6–7 weeks | Transabdominal USG | Heart rate may be detected (less sensitive) | | 8+ weeks | Both modalities | Robust, easily identifiable cardiac motion | **High-Yield:** FCA should be present when the gestational sac reaches a mean diameter of 16–20 mm on transvaginal ultrasound. Absence of FCA at this threshold raises concern for miscarriage or ectopic pregnancy. ### Clinical Correlation **Clinical Pearl:** In early pregnancy dating, the presence of FCA at 5–6 weeks confirms viability and allows accurate dating to within ±3–5 days. This is more accurate than dating by crown-rump length (CRL) in the second trimester (±2 weeks) or abdominal circumference in the third trimester (±4 weeks). **Warning:** Do not confuse the **yolk sac** (normally visible at 5–6 weeks, ~3–6 mm) with fetal cardiac activity. The yolk sac is a non-contractile structure; FCA requires visualization of rhythmic motion within the fetal heart. [cite:Callen Obstetric Ultrasound Ch 3] 
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