Formula:
The Mean Sac Diameter (MSD) is a crucial ultrasound measurement used in early pregnancy to estimate gestational age. It is calculated by averaging three perpendicular dimensions of the gestational sac, providing a reliable indicator before the appearance of the fetal pole.
How is Mean Sac Diameter (MSD) Calculated?
MSD = (Length + Width + Height) / 3
Estimating Gestational Age (GA) from MSD
Once the Mean Sac Diameter (MSD) is determined, the estimated gestational age (GA) can be derived using the following common formulas:
GA (days) = MSD (mm) + 30
GA (weeks) = (MSD (mm) + 30) / 7
Variable Explanation:
- Length: The longest visible dimension of the gestational sac (in millimeters).
- Width: A dimension perpendicular to the length of the gestational sac (in millimeters).
- Height: The third dimension, perpendicular to both length and width, of the gestational sac (in millimeters).
- MSD: Mean Sac Diameter, the average of the three measurements (in millimeters).
- GA: Estimated Gestational Age, provided in days and weeks.