Mean Sac Diameter (MSD) Medically Reviewed

Last Updated: June 10, 2020

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

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.

FertilitySmarts explains Mean Sac Diameter (MSD)

MSD can be measured between5-6 weeks and 11 weeks but it isn’t recommended once the embryo is visible and can be assessed.

The mean sac diameter can be measured on either a transvaginal or abdominal ultrasound.

  • Transvaginal ultrasound from 5 weeks from last menstrual period (LMP)
  • Transabdominal ultrasound from 6 weeks last menstrual period (LMP)

How is mean sac diameter measured?

MSD is measured by adding together the height, length, and width of the gestational sac and then dividing the result by 3 and expressed in millimeters.

How is gestational age estimated with MSD?

Gestational age can be estimated by adding 30 to the MSD. The result is GA in days.

How is MSD used?

Ultrasound technicians measure the MSD and look for certain structures within the gestational sac that should be present. For example,

  • When the MSD is 5 mm, it is possible to see the yolk sac.
  • A smaller than expected sac around 7- 8 weeks (but not before) indicates that the pregnancy is not progressing normally and will end in miscarriage.
  • Typically a healthy gestation is considered one in which the MSD is at least 5 mm greater than the crown-rump length (CRL).
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