## Gestational Sac Diameter and Dating in Early Pregnancy ### Ultrasound Dating in First Trimester **Key Point:** The gestational sac (GS) is the first sonographic evidence of intrauterine pregnancy, appearing at approximately 4.5–5 weeks of gestation (mean hCG ~1,000–2,000 mIU/mL). It grows at a predictable rate of approximately 1 mm per day in the early weeks. ### Gestational Sac Diameter to Dating Conversion | GS Diameter (mm) | Gestational Age (weeks) | Accuracy | |---|---|---| | 2–3 | 4–5 | ±3 days | | 4–5 | 5–6 | ±3 days | | 6–7 | 5.5–6.5 | ±3 days | | 8–10 | 6–7 | ±3 days | | 11–14 | 7–8 | ±3–5 days | | 15–20 | 8–9 | ±5 days | **High-Yield:** An 8 mm gestational sac corresponds to approximately **4–5 weeks of gestation** (mean ~4.5 weeks). The absence of a yolk sac at this size is normal; the yolk sac typically becomes visible when the GS diameter reaches 5–6 mm (around 5–5.5 weeks). ### Clinical Correlation **Clinical Pearl:** The patient's reported amenorrhea of 6 weeks is calculated from the last menstrual period (LMP), which is conventionally 2 weeks before ovulation. Therefore, her actual gestational age by dating is approximately 4–5 weeks, which aligns with the ultrasound findings. **Mnemonic:** **GS-AGE** — Gestational Sac size in mm ≈ Gestational Age in weeks (roughly 1 mm = 1 week after 4 mm, but always refer to standard charts). ### Why This Matters for Dating 1. First-trimester ultrasound dating (before 14 weeks) is accurate to within **±3–5 days**. 2. Dating by GS diameter is most reliable between 4–8 weeks. 3. By the second trimester, dating accuracy decreases to ±1–2 weeks; by the third trimester, ±3–4 weeks. **Warning:** Do not confuse gestational age (from LMP or ultrasound) with embryonic age (from conception), which is 2 weeks less.
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