Mean Sac Diameter Calculator (MSD to gestational age, early pregnancy)

Understanding the mean sac diameter calculator — concept and principle

The gestational sac is the first structure seen on an early pregnancy ultrasound. This calculator averages its three measurements into the mean sac diameter (MSD), estimates gestational age and due date from it, and shows how the size relates to the Society of Radiologists in Ultrasound (SRU) 2013 criteria when no embryo is seen.

Average of three diameters: with the default inputs the result is 6 weeks 6 days (due date 30 Jun 2027).

How it is calculated

MSD (mm) = (length + width + height) ÷ 3

Gestational age (days) ≈ MSD + 30

Due date = scan date − gestational age (days) + 280

Measurements are inner diameters in three perpendicular directions.
Once an embryo is visible, its crown-rump length (CRL) dates the pregnancy more accurately.

The principle

In the first weeks the sac grows by roughly 1 mm a day, so adding a constant of 30 days to the MSD gives a usable estimate of gestational age from about 5 to 7 weeks. Averaging three diameters smooths out the sac's irregular shape. The SRU panel set conservative cut-offs so that a wanted pregnancy is never wrongly declared lost.

Worked example

Default inputs: length 20, width 16, height 18 mm, embryo seen, scan 10 Nov 2026.

MSD = (20 + 16 + 18) ÷ 3 = 18 mm.
Gestational age = 18 + 30 = 48 days = 6+6 weeks.
Due date = 10 Nov 2026 − 48 + 280 days = Wed 30 Jun 2027.
Embryo seen → use the CRL and heartbeat.

Each point is the calculator re-run with only “Sac length (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Sac length (mm)	Mean sac diameter (mm)
5	13
10	14.7
15	16.3
20	18
25	19.7
30	21.3
40	24.7

How to read the result

When an embryo is seen, dating and viability are judged from the CRL and heartbeat. With no embryo, an MSD of 25 mm or more meets a criterion for pregnancy loss; an MSD of 16–24 mm is only suspicious, and a repeat scan after 7–10 days is advised. Below 16 mm no size criterion applies.

Common mistakes

Measuring outer rather than inner diameters.
Dating by MSD when a CRL is available.
Deciding about a wanted pregnancy from a single number.

Keep in mind: This is a reference for understanding your report, not a diagnosis. Diagnosis of pregnancy loss depends on the full scan and often a repeat scan.

Frequently asked questions

What MSD is normal at 6 weeks?

Roughly 12–15 mm by this rule, but individual sacs vary.

When should an embryo be seen?

By SRU criteria, absence of an embryo with an MSD of 25 mm or more indicates pregnancy loss.

Is the due date from MSD accurate?

It is a rough estimate; CRL dating is more precise.

Sources: Doubilet PM, Benson CB, Bourne T, et al., "Diagnostic criteria for nonviable pregnancy early in the first trimester", N Engl J Med 2013;369:1443–1451 · ACOG Practice Bulletin No. 150, "Early Pregnancy Loss" (2015)

Checked 9 Nov 2026. MSD = mean of three diameters and “MSD (mm) + 30 = days of pregnancy” as given in Radiopaedia, “Mean sac diameter” (citing Chudleigh & Thilaganathan, Obstetric Ultrasound, 2004); size criteria from Doubilet PM, Benson CB, Bourne T, et al., N Engl J Med 2013;369:1443–1451, as summarised with ACOG Practice Bulletin 150 (2015) by MAHEC “Early pregnancy viability US criteria”.

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