A 28-year-old nulliparous woman presents to the antenatal clinic with a history of amenorrhea for 6 weeks. She reports regular 28-day cycles previously. On examination, her uterus is palpable just above the symphysis pubis. A urine pregnancy test 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 gestational age at this presentation?

## Gestational Age Calculation from Gestational Sac Diameter **Key Point:** The gestational sac (GS) diameter is the most reliable ultrasound parameter for dating pregnancy in the first trimester, with an accuracy of ±3–5 days when measured between 8–13 weeks. ### Gestational Sac Measurement Formula The relationship between gestational sac diameter (in mm) and gestational age is: $$\text{Gestational Age (weeks)} = \frac{\text{GS diameter (mm)} + 2}{7}$$ Alternatively, a commonly used approximation: - GS diameter of 2–3 mm → 4 weeks + 3 days to 5 weeks - GS diameter of 5–6 mm → 5 weeks + 3 days to 5 weeks + 6 days - GS diameter of 8 mm → **5 weeks + 4 days** - GS diameter of 10 mm → 6 weeks - GS diameter of 16 mm → 7 weeks ### Clinical Correlation In this case: - **Gestational sac diameter = 8 mm** - **Calculated gestational age = 5 weeks + 4 days** - The absence of a yolk sac at 8 mm GS diameter is normal; the yolk sac typically becomes visible when GS diameter reaches 5–6 mm, but may not always be clearly seen at 8 mm depending on ultrasound quality and orientation. **High-Yield:** The mean sac diameter (MSD) is preferred over single diameter measurements. When MSD is 8 mm, gestational age is approximately 5 weeks + 4 days, with a confidence interval of ±3–5 days. **Clinical Pearl:** Amenorrhea of 6 weeks (from last menstrual period) correlates with a gestational age of approximately 6 weeks by menstrual dating, but ultrasound dating (5 weeks + 4 days) may differ by a few days due to variation in ovulation timing and cycle length. The ultrasound measurement is more accurate in the first trimester. ### Why Yolk Sac Is Absent The yolk sac typically becomes visible at a GS diameter of 5–6 mm (around 5 weeks + 2 days). At 8 mm, it should usually be visible, but its absence does not indicate abnormality if the GS is small and the ultrasound resolution is limited. A follow-up scan in 1 week is recommended if there is diagnostic uncertainty.