🩺 Mean Sac Diameter Calculator
Calculate the mean sac diameter from an early ultrasound and see the estimated gestational age it corresponds to.
What mean sac diameter shows
The mean sac diameter (MSD) is an early ultrasound measurement of the gestational sac, taken before a fetal pole is visible, typically between about 5 and 8 weeks of pregnancy. It is calculated by averaging three measurements of the sac taken in perpendicular directions: length, width, and height.
How this calculator works
Enter the three sac measurements in millimeters from your ultrasound report. The calculator averages them to find the MSD, then applies a widely used quick-reference formula, gestational age in days equals MSD in millimeters plus 30, which is considered reasonably accurate for sacs measuring roughly 2 to 24 millimeters.
Once a fetal pole and heartbeat are visible, crown-rump length becomes a more accurate way to date a pregnancy than sac size alone. This calculator is for general educational purposes only and results should be interpreted by your healthcare provider alongside the full ultrasound findings.