Mean Sac Diameter Calculator

Average three gestational sac measurements to get the MSD, estimate gestational age and due date using the MSD+30 rule, and check the result against standard early-pregnancy viability criteria.

Quick Facts

MSD formula
(D1 + D2 + D3) / 3
Average of three orthogonal sac diameters (length, height, width), each measured inner wall to inner wall.
Dating rule
GA (days) = MSD (mm) + 30
Robinson's rule of thumb; most reliable for MSD 2-24 mm, before a fetal pole is visible.
Failure criterion
MSD 25 mm or larger, no embryo
Diagnostic of pregnancy failure per the 2013 SRU/ACR consensus; 16-24 mm without an embryo is suspicious but not diagnostic.

Your Results

Calculated
Mean sac diameter
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Average of 3 diameters (mm)
Estimated gestational age
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MSD + 30 days rule
Estimated due date
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40 weeks from estimated LMP
Viability screen
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Based on MSD and scan findings

Ready

Enter the three sac diameters, the scan date, and what was seen on the scan, then calculate.

About the mean sac diameter

The mean sac diameter (MSD) is an early-pregnancy ultrasound measurement used before a fetal pole is visible, typically during the fifth and sixth weeks of gestation. A sonographer measures the gestational sac in three planes - length, height, and width - from inner wall to inner wall, and the mean sac diameter is simply the average of those three numbers.

How the formula works

MSD (mm) = (diameter 1 + diameter 2 + diameter 3) / 3. Because the sac is rarely a perfect sphere, averaging three orthogonal measurements smooths out the effect of measuring a slightly oval shape from a single angle.

Estimating gestational age from the MSD

Before a crown-rump length (CRL) can be measured, sonographers commonly use a simple rule of thumb known as Robinson's rule: gestational age in days is approximately equal to the MSD in millimeters plus 30. This estimate is most reliable for an MSD between about 2 and 24 mm; once the sac is larger and an embryo is visible, CRL-based dating is more accurate and should be used instead. The estimated due date on this page adds the remaining days of a standard 280-day (40-week) pregnancy to the scan date you enter.

Using the MSD to screen for early viability

The Society of Radiologists in Ultrasound and the American College of Radiology published consensus criteria (Doubilet et al., 2013) for distinguishing a normal early pregnancy from a failed one. An MSD of 25 mm or more with no visible embryo is considered diagnostic of pregnancy failure. An MSD of 16-24 mm with no embryo is suspicious but not diagnostic on its own, and the recommended next step is a follow-up scan in 7 to 14 days rather than an immediate conclusion. Below these thresholds, an empty-appearing sac is frequently just early, since a yolk sac is not always visible until the MSD reaches roughly 8 mm and an embryo not until roughly 16-18 mm.

When to consult a professional

This calculator performs the standard MSD arithmetic and applies published dating and viability thresholds; it does not read the ultrasound images themselves and cannot replace a scan interpreted by a qualified sonographer or physician. Any concern about pregnancy viability should be discussed directly with your obstetric care provider, who can weigh serial scans, hCG trends, and your full clinical picture.

Frequently Asked Questions

What is the mean sac diameter (MSD) and how is it measured?
The mean sac diameter is the average of three ultrasound measurements of the gestational sac - length, height, and width - each taken from the inner wall of the sac. It is used in very early pregnancy, before a fetal pole can be measured, roughly between the fifth and sixth weeks of gestation.
How is gestational age estimated from the MSD?
A common rule of thumb (Robinson's rule) estimates gestational age in days as the MSD in millimeters plus 30. This works best for an MSD of about 2 to 24 mm. Once an embryo is visible, crown-rump length gives a more accurate estimate and should be used instead.
What does the MSD indicate about pregnancy viability?
Published consensus criteria consider an MSD of 25 mm or more with no visible embryo diagnostic of pregnancy failure, while 16 to 24 mm with no embryo is suspicious but not diagnostic and usually warrants a follow-up scan in 7 to 14 days. Smaller sacs without a yolk sac or embryo are frequently just early. These thresholds are a starting point for discussion with a care provider, not a stand-alone diagnosis.

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