Mean Sac Diameter (MSD) Gestational Age Calculator
Calculate mean gestational sac diameter (MSD) from three ultrasound measurements, estimate gestational age in early pregnancy and see the SRU size criteria for an empty sac.
How it works
MSD (mm) = (length + width + height) ÷ 3 · gestational age (days) ≈ MSD (mm) + 30
In early pregnancy, before an embryo can be measured, the gestational sac grows by about 1 mm a day, so gestational age in days is roughly the mean sac diameter in millimetres plus 30 (a rule widely used in early-pregnancy units, valid at about 5–7 weeks). The three diameters are measured inside the sac's fluid space in two perpendicular planes. Once an embryo is visible, crown–rump length dates the pregnancy more accurately. The Society of Radiologists in Ultrasound (Doubilet et al., 2013) consensus uses MSD to judge an empty sac: 25 mm or more with no embryo is diagnostic of pregnancy failure, and 16–24 mm is suspicious.
Worked example
A sac measuring 20 × 15 × 16 mm has MSD = (20 + 15 + 16) ÷ 3 = 17.0 mm, giving a gestational age of about 17 + 30 = 47 days = 6 weeks 5 days.
Sources: Doubilet PM et al. (2013) Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med 369:1443–1451 · Radiopaedia — Mean sac diameter
Frequently asked questions
How is the mean sac diameter measured?
The sonographer measures the inner fluid space of the gestational sac in three perpendicular directions — length and height on a sagittal view, width on a transverse view — and averages them.
How accurate is dating by sac size?
Less accurate than crown–rump length, roughly ±1 week. It is used mainly before about 6 weeks when no embryo is measurable, and the MSD + 30 rule becomes less reliable above about 25 mm.
What does an empty sac of 25 mm mean?
According to the SRU consensus, an MSD of 25 mm or more without an embryo is diagnostic of a failed pregnancy. Sizes of 16–24 mm without an embryo are only suspicious and a repeat scan in 7–14 days is usually recommended. Decisions must be made with your clinician.
When should a yolk sac and embryo appear?
A yolk sac is usually seen once MSD is about 8–10 mm, and an embryo with a heartbeat by around 6–7 weeks. Uncertain dates are common, which is why follow-up scans are used rather than single measurements.
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This calculator is for education only and does not replace your obstetric care team. Ultrasound measurements, their interpretation and any dating or diagnosis must come from a qualified sonographer or clinician.