Comprehensive Calculator Guide
This calculator works out the mean gestational sac diameter (MSD) from the sac’s three measurements on the scan and estimates gestational age from it in the first weeks, before the embryo itself can be measured. A 12 × 10 × 14 mm sac averages 12 mm, about 42 days — 6 weeks. It’s a rough estimate, about ±5 days.
The diameter-plus-30 rule and its limits
The sac shows on a transvaginal scan around week 5 and grows about a millimetre a day, so age in days ≈ MSD in mm + 30. MSD is the average of three perpendicular diameters.
The rule only suits the first weeks; once the embryo is visible, its crown-rump length is more accurate. Doctors read sac size with what’s inside it and may repeat the scan before drawing any conclusion.
How to Use
- Enter the sac’s three diameters in mm from the scan report.
- Enter the scan date, or leave it empty for today.
- Press “Work out the dates” to see the MSD, gestational age and approximate due date.
Formula Used
MSD = (length + width + height) ÷ 3 | Gestational age (days) ≈ MSD (mm) + 30
Practical Examples
- Example 1
- 12 × 10 × 14 mm: MSD 12, 42 days (6+0).
- Example 2
- 20 × 22 × 21 mm: MSD 21, 51 days (7+2).
Important Tips
- Compare with dating from your last period; small gaps are normal.
Common Mistakes to Avoid
- Using the rule once the embryo can be measured.
Frequently Asked Questions
- How do I estimate gestational age from the sac?
- Add 30 to the mean sac diameter in mm.
- What is MSD on an ultrasound?
- The mean gestational sac diameter, the average of its three measurements.
- How fast does the sac grow?
- About a millimetre a day in the first weeks.