Mean sac diameter

Mean sac diameter (MSD) is a sonographic measurement of the gestational sac, which is usually first seen at around 3 weeks after conception (5 weeks after the last menstrual period), when it measures 2-3 mm.

Measurement

MSD = (length + height + width)/3

Normal MSD (in mm) + 30 = days of pregnancy

Interpretation

For a healthy gestation, it is suggested that the mean sac diameter should be at least 5 mm greater than the crown rump length.

The MSD increases by about 1 mm per day, however, there is considerable variation in the rate of MSD growth, with overlap between viable and non-viable pregnancies. The diagnosis of pregnancy failure should not be made on the basis of MSD growth ⁴.

Transvaginal ultrasound

• MSD of >25 mm with absent fetal pole indicates pregnancy failure (missed miscarriage) 
• an MSD of 16-24 mm without an embryo is suspicious for pregnancy failure, but not definitive.
• when the MSD measures 8 mm a yolk sac should be visible, however ,lack of a yolk sac is not an indication of pregnancy failure

Transabdominal ultrasound

• when the MSD measures ≥25 mm a fetal pole should be visible
• when the MSD measures 20 mm a yolk sac should be visible
  • lack of a yolk sac is not a definite indication of pregnancy failure

See also 

• small gestational sac