Mean Sac Diameter Calculator

What Is Mean Sac Diameter (MSD)?

Mean sac diameter (MSD) is the average of three orthogonal measurements of the gestational sac on early pregnancy ultrasound: length, width, and height. MSD is used in the first trimester to estimate gestational age before crown-rump length (CRL) can be measured reliably. It also helps confirm intrauterine pregnancy and assess early pregnancy viability. Combine results with the Luteal Phase Calculator for cycle timing context and the IVF Due Date Calculator for assisted reproduction dating.

How MSD Is Calculated

MSD = (length + width + height) / 3, with all three dimensions measured in millimeters on transvaginal or transabdominal ultrasound. Gestational age in days is estimated as MSD (mm) + 30, then converted to weeks and days. For example, an MSD of 10 mm corresponds to approximately 40 days, or 5 weeks 5 days. Accuracy is typically within about 5 days in the early first trimester when the sac is clearly visible.

MSD and Gestational Age Interpretation

A gestational sac becomes visible on transvaginal ultrasound when MSD reaches approximately 2 to 3 mm, usually around 4 to 5 weeks gestation. MSD up to 30 mm remains within the range where sac measurement is useful for dating. Beyond 30 mm, crown-rump length is preferred because it is more precise. An MSD that is too small for expected dates, or an empty sac without a yolk sac or embryo, may indicate early pregnancy loss and requires clinical follow-up.