A 28-year-old married woman from Delhi presents to the antenatal clinic with amenorrhea for 6 weeks. She reports breast tenderness and mild nausea in the mornings. On examination, her uterus is palpable just above the symphysis pubis and is soft with a bluish tinge to the cervix. A urine pregnancy test (hCG) is positive. Transvaginal ultrasound shows a gestational sac of 8 mm diameter in the uterine cavity with no yolk sac visible. What is the most accurate assessment of gestational age based on the gestational sac diameter?

Question

Accepted Answer

C. 4–5 weeks of gestation. ## 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.

Answer

A. 6–7 weeks of gestation

Answer

B. 10–11 weeks of gestation

Answer

D. 8–9 weeks of gestation

Explanation

Gestational Sac Diameter and Dating in Early Pregnancy

Ultrasound Dating in First Trimester

Gestational Sac Diameter to Dating Conversion

Clinical Correlation

Why This Matters for Dating

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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