Enter gestational sac length, width, and height to get the mean sac diameter (MSD) and an early gestational age estimate. You can also enter a single MSD and add a scan date for context.
How the estimate is calculated
- MSD measurement: the calculator averages three orthogonal sac diameters (length, width, height). If a direct MSD is provided, that value is used.
- MSD → GA rule of thumb: early first-trimester GA in days is approximated by GA ≈ MSD(mm) + 30. This reflects that the gestational sac grows about 1 mm per day in very early pregnancy.
- Typical range: a variability band of about ±3 days is shown to reflect inter-observer and biological variation in early measurements.
- Units: cm inputs are converted to mm before the MSD average and GA estimate are computed.
- Dates: if a scan date is entered, it is displayed for context; it does not alter the GA from MSD calculation.
Why MSD has uncertainty
- Operator & plane selection: small changes in caliper placement or section plane can shift the MSD by 1–2 mm, which is 1–2 days in the estimate.
- Biologic variation: implantation timing varies; two pregnancies with the same MSD can be a few days apart in true GA.
- Very early scans: below ~10 mm MSD, rounding and sac shape irregularity increase spread; a wider clinical context is important.
Follow-up (educational)
- Re-scan interval: early pregnancy units commonly review in about 7–14 days when dates are uncertain or findings are borderline.
- Confirm with CRL when visible: once an embryo and crown-rump length are measurable, CRL-based dating is preferred.
Limitations
- GA from MSD is an approximation for early first trimester and is not used to declare nonviability by itself.
- Maternal factors, multiple gestation, and scan equipment can influence measurements.
- Radiopaedia: Mean sac diameter — includes the common MSD(mm) + 30 days approximation and measurement technique.
- Doubilet & Benson. First-Trimester Ultrasound. NEJM 2013 — review of early ultrasound, sac growth expectations, and viability criteria.
- AIUM Practice Parameter for the Performance of Obstetric Ultrasound — measurement standards and early pregnancy guidance.
- Radiopaedia: Gestational sac — technique notes and normal early growth patterns.
How to use this calculator
- Enter sac length, width, and height (mm or cm). The tool converts units.
- Optional: enter a scan date to display an EDD alongside GA (your app may show this next to results).
- Optional: if you already have an MSD, use Direct MSD entry.
- Read the outputs: mean sac diameter (mm) and estimated gestational age (days or weeks+days) with a typical range.
MSD is useful very early. Once a CRL is measurable, CRL-based dating is usually preferred. Ultrasound reports guide clinical decisions.
Last updated: September 21, 2025
Frequently asked questions
What is mean sac diameter (MSD)?
MSD is the average of the gestational sac length, width, and height measured on ultrasound. It summarizes sac size early in pregnancy.
How is MSD calculated from three measurements?
The calculator takes the three sac dimensions and computes their average, returning MSD in millimeters. You can enter values in mm or cm.
How accurate is MSD for dating compared with CRL?
MSD gives an early estimate before CRL is visible. Once a crown–rump length can be measured, CRL-based dating is usually more precise. See the CRL → GA & EDD calculator.
Can I enter a single MSD value instead of three dimensions?
Yes. Use the direct MSD input to estimate gestational age without entering length, width, and height separately.
My scan showed a sac but no embryo yet—what next?
Very early scans may show a sac before a yolk sac or embryo is visible. Many clinics repeat ultrasound in about 1–2 weeks. For timing context, see hCG Levels & Doubling and the Implantation Window.
Does this work for twins?
Each gestational sac is measured separately. Dating and follow-up are based on ultrasound findings for each sac.