Mean Sac Diameter (MSD)

Definition - What does Mean Sac Diameter (MSD) mean?

Mean sac diameter (MSD) is an ultrasound measurement of the gestational sac. The gestational sac contains the fetus and amniotic fluid. It is usually first seen at 5 weeks of gestation. The gestational sac's growth and development is associated with the growth and development of the fetus. Because of this, MSD can help predict the viability of a pregnancy and can aid in the diagnosis of pregnancy failure (missed miscarriage).

FertilitySmarts explains Mean Sac Diameter (MSD)

The mean sac diameter can be measured on either a transvaginal or abdominal ultrasound. It is measured by adding together the height, length, and width of the gestational sac and then dividing the result by 3.

Normally MSD increases at a rate of about 1mm per day. However, there is some variability in this number. Typically a healthy gestation is considered one in which the MSD is at least 5mm greater than the crown-rump length.

Ultrasound technicians measure the MSD and look for certain structures within the gestational sac that should be present. For example, when the MSD is 20mm on an abdominal ultrasound, the yolk sac should be visible. Although abnormalities in these findings may be indicative of fetal demise, this measurement alone should not be used to diagnose pregnancy failure.

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