Compute mean sac diameter (MSD) from your early-pregnancy ultrasound's three gestational-sac measurements, estimate gestational age with the classic MSD + 30 rule, and check the 25 mm discriminatory-zone criterion from Society of Radiologists in Ultrasound guidelines.
Enter the gestational sac's length, width, and height from your ultrasound report (inner edge to inner edge). MSD = (length + width + height) ÷ 3.
Enter a known mean sac diameter. The classic first-trimester estimate is: gestational age (days) ≈ MSD (mm) + 30 (Hellman et al., 1969). Typical uncertainty is about ±5 days.
Compare an MSD measurement against the Society of Radiologists in Ultrasound discriminatory criteria for early pregnancy assessment. This is guideline information, not a diagnosis.
Measure the gestational sac in three perpendicular (orthogonal) planes — length, width, and height — from inner edge to inner edge of the sac wall, then average them: MSD = (length + width + height) / 3. For example, sac dimensions of 20 mm, 22 mm, and 18 mm give an MSD of 20.0 mm. All three measurements should be in the same unit (millimetres are standard).
The classic first-trimester rule of thumb is: gestational age in days ≈ MSD in millimetres + 30. This relationship was described by Hellman and colleagues in 1969 and reflects that the gestational sac grows roughly 1 mm per day in very early pregnancy. For example, an MSD of 20 mm estimates about 50 days (7 weeks 1 day). It is most reliable between about 4 and 10 weeks, with a typical uncertainty of ±5 days.
Crown-rump length (CRL) is better. Once an embryo with a measurable CRL is visible (usually from about 6–7 weeks), CRL dating is more accurate than MSD-based dating and should be used instead. MSD is mainly useful in the very early window when the sac is the only structure visible on ultrasound.
Guidelines from the Society of Radiologists in Ultrasound (Doubilet et al., New England Journal of Medicine 2013) and the ASA Clinical Guideline (May 2024) list a mean sac diameter of ≥ 25 mm without a yolk sac or embryo as a criterion suggestive of early pregnancy failure on transvaginal ultrasound. Below 25 mm with no yolk sac or embryo, findings are considered suspicious and require a follow-up scan at least 14 days later — never a conclusion from one scan.
No — not by itself, and no online calculator can make that determination. A mean sac diameter ≥ 25 mm with no yolk sac or embryo is a guideline criterion that is suggestive of early pregnancy failure, but it must be confirmed by a clinician with proper clinical context and, when needed, repeat imaging. Diagnosis of pregnancy loss has serious consequences, so guidelines are intentionally conservative.
Exact timing varies between pregnancies and depends on scan technique (transvaginal is more sensitive than transabdominal). As a rough guide, the yolk sac often becomes visible when the MSD is in the mid-teens of millimetres, and an embryo shortly after. Because of this variation, the follow-up-interval criteria (at least 14 days when only a sac was seen, at least 11 days when a yolk sac was seen) matter more than any single size cutoff — discuss your specific scan with your clinician.
Yes. Select centimetres in the unit dropdown and the calculator converts each dimension to millimetres (1 cm = 10 mm) before averaging. For example, 2.0 cm, 2.2 cm, and 1.8 cm become 20 mm, 22 mm, and 18 mm, giving an MSD of 20.0 mm.
No. This is an educational tool that reproduces published ultrasound formulas and guideline criteria. It cannot diagnose viability, date a pregnancy definitively, or replace your obstetrician, midwife, or sonographer. If you have bleeding, severe pain, dizziness, or any concern about your pregnancy, contact your healthcare provider or seek urgent care.
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