Gestational Sac Assessment
Measure the gestational sac in three orthogonal planes (length, width, height) and average them to determine the mean sac diameter (MSD).
How to Use This Calculator
Obtain accurate measurements
Use a transvaginal ultrasound and measure the gestational sac in three perpendicular planes internally (inner-to-inner).
Input all dimensions
Length, width, and height are averaged to reduce measurement error and estimate gestational age.
Compare with clinical dating
Identify significant discrepancies and plan follow-up ultrasounds or viability assessments accordingly.
Formula
Mean sac diameter (MSD) = (Length + Width + Height) ÷ 3
Estimated gestational age (days) = MSD (mm) + 30
Gestational age (weeks + days) derived by dividing total days by 7
Pregnancy failure criteria: MSD ≥25 mm without embryo or crown-rump length ≥7 mm without cardiac activity (SRU 2013)
Full Description
Early gestational sac measurements aid in viability assessment before the embryo and cardiac activity are visible. The mean sac diameter correlates with gestational age, though crown-rump length becomes more precise once the embryo is seen. If MSD reaches 25 mm without an embryo, the Society of Radiologists in Ultrasound recommends diagnosing pregnancy failure. Serial ultrasounds are key to avoid false-positive diagnoses.
Frequently Asked Questions
Why average three measurements?
Gestational sacs are often elliptical. Averaging length, width, and height reduces measurement variability.
When should I repeat the scan?
If no embryo is seen, repeat in 7–10 days to look for growth and new structures (yolk sac, fetal pole).
What if there is a yolk sac but no embryo?
Presence of a yolk sac without embryo is normal up to about 5.5–6 weeks. Monitor growth trends closely.
How accurate is MSD for dating?
MSD is less precise than crown-rump length. Once fetal pole appears, switch to CRL for gestational dating.