Last updated October 3, 2026 · Report an error
Averages three gestational sac diameter measurements (mm) from an early ultrasound and estimates gestational age using the standard published regression formula, commonly cited as the "MSD + 30" rule from the early gestational sac dating literature: GA (days) = MSD (mm) + 30. Most reliable in the very early first trimester (roughly 5-6 weeks) before a crown-rump length can be measured. Same framing as crown-rump length dating — an estimate to discuss with a healthcare provider, not a replacement for clinical dating. For general reference and educational purposes only.
How it works
Enter three gestational sac diameter measurements in millimeters from an ultrasound report. The tool averages them, then applies the MSD+30 formula.
- Enter sac diameter measurement 1 (mm).
- Enter sac diameter measurement 2 (mm).
- Enter sac diameter measurement 3 (mm).
- Click Calculate to see your results.
Examples
Measurements 18, 19, 17 mm
MSD = (18+19+17)/3 = 18 mm. GA = 18 + 30 = 48 days ≈ 6 weeks, 6 days.
Who should use it
- Reviewing a very early ultrasound report alongside a healthcare provider.
- Obstetric education about first-trimester dating methods.
Industry applications
- Obstetric ultrasound reference
- Early pregnancy clinic education
Advantages
- Usable earlier in pregnancy than crown-rump length.
- Simple, standard formula.
Limitations
- Less precise than CRL dating.
- Only useful in the very early first trimester.
Common mistakes to avoid
- Using MSD dating alone when a CRL measurement is also available (CRL is generally preferred once measurable).
- Averaging measurements taken in inconsistent planes.
Best practices
- Use all three orthogonal sac diameter measurements from the ultrasound report for the most representative average.
Tips
- A follow-up scan with a measurable CRL will typically give a more precise gestational age estimate.