The gestational sac is the first structure seen on ultrasound, from about 4.5–5 weeks. It grows about 1 mm a day in early pregnancy. Diagnosis of pregnancy loss is never made from a single number without careful follow-up.
How to use the Mean Sac Diameter Calculator
- Length
- Width
- Height
- Embryo seen?
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the three perpendicular sac diameters measured from inner edge to inner edge, and whether an embryo is seen.
Inputs explained
| Field | What to enter |
|---|---|
| Length | In mm |
| Width | In mm |
| Height | In mm |
| Embryo seen? | Choose one: Yes, No |
What each result means
- Mean sac diameter = ([Length] + [Width] + [Height])÷3
- Estimated gestational age = [Mean sac diameter] + 30
- Gestational age – weeks = floor([Estimated gestational age]÷7)
- …plus days = mod(round([Estimated gestational age]), 7)
Example values
How the main result changes when Length changes and everything else stays at the example values above:
| Length (mm) | Gestational age – weeks (weeks) |
|---|---|
| 10 | 6 |
| 15 | 6 |
| 20 | 6 |
| 25 | 7 |
| 30 | 7 |
| 40 | 7 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
MSD (mm) = (length + width + height) ÷ 3
Gestational age (days) ≈ MSD + 30
No embryo with MSD ≥ 25 mm is diagnostic of pregnancy failure (SRU 2013)
Worked example
Inputs — Length: 20 mm; Width: 16 mm; Height: 18 mm; Embryo seen?: Yes.
Results — Mean sac diameter = 18 mm; Estimated gestational age = 48 days; Gestational age – weeks = 6 weeks; …plus days = 6 days; Interpretation (SRU criteria) = No concerning finding.
Key terms
- MSD
- Mean sac diameter.
Frequently asked questions
When is the gestational sac seen?
From about 4.5–5 weeks with a transvaginal scan.
How fast does the sac grow?
About 1 mm per day.
Does an empty sac always mean miscarriage?
Not if it is small; a repeat scan is needed before any diagnosis.