## Earliest Sonographic Signs of Intrauterine Pregnancy **Key Point:** The **gestational sac (GS)** is the earliest sonographic sign of intrauterine pregnancy, appearing at approximately 4.5–5 weeks of gestation (mean sac diameter ~2–3 mm). ### Sequential Appearance of Pregnancy Landmarks | Gestational Age | Sonographic Finding | Mean Sac Diameter (MSD) | | --- | --- | --- | | 4.5–5 weeks | Gestational sac | 2–3 mm | | 5–6 weeks | Yolk sac | 3–6 mm | | 5–6 weeks | Fetal pole | ~5 mm | | 5–6 weeks | Fetal cardiac activity | Present | | 8–10 weeks | CRL measurement | Accurate dating ±3–5 days | **High-Yield:** The **double decidual sign** (two echogenic rings representing the gestational sac surrounded by decidua) confirms intrauterine location and helps exclude pseudosac (fluid collection in the endometrial cavity mimicking GS in ectopic pregnancy). ### Mnemonic: **GYF-C** **G** — Gestational sac (earliest, ~4.5–5 weeks) **Y** — Yolk sac (follows GS, ~5–6 weeks) **F** — Fetal pole (appears with yolk sac, ~5–6 weeks) **C** — Cardiac activity (confirms viability, ~5–6 weeks) **Clinical Pearl:** A gestational sac with MSD ≥16–20 mm without a yolk sac or fetal pole is concerning for either incorrect dating or a failing pregnancy (blighted ovum). Serial ultrasound and beta-hCG levels help differentiate. **Warning:** Do not confuse a **pseudosac** (fluid collection in the endometrial cavity seen in ectopic pregnancy) with a true gestational sac. The pseudosac is centrally located, lacks the double decidual sign, and does not grow appropriately on serial ultrasound. [cite:Callen Obstetric Ultrasound Ch 3] 
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