Mean sac diameter (MSD) is an ultrasound measurement of the structure that houses a developing embryo in very early pregnancy called the gestational sac. It is usually first seen with transvaginal ultrasound at around 5 weeks gestation.

The gestational sac is the first visible structure in a pregnancy, and can be used to assess the viability of the pregnancy, diagnose an early miscarriage, and estimate the gestational age.