This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Dating a very early pregnancy
The gestational sac is the first structure seen on an early ultrasound, usually from about 4.5 to 5 weeks after the first day of the last period with a transvaginal probe. Before the embryo can be measured, the size of the sac gives a rough estimate of gestational age. The mean sac diameter (MSD) is the average of three perpendicular measurements of the fluid-filled space:
MSD = (length + height + width) ÷ 3
Gestational age
A widely used rule of thumb is:
Gestational age (days) ≈ MSD (mm) + 30
So a sac of 10 mm corresponds to about 40 days, or 5 weeks and 5 days. The sac grows about 1 mm a day in early pregnancy. The estimate is less precise than the crown-rump length, which dates a pregnancy to within about 5 to 7 days and replaces the MSD as soon as the embryo is visible.
Due date
The calculator counts the due date as 280 days, or 40 weeks, from the dated start of pregnancy, as in the US. In France, the reference term is 41 weeks.
What the size can mean
In 2013 a panel of the Society of Radiologists in Ultrasound published criteria to avoid misdiagnosing an early loss in a pregnancy that is still developing:
| Finding | Interpretation |
|---|---|
| MSD of 25 mm or more and no embryo | diagnostic of pregnancy loss |
| MSD of 16 to 24 mm and no embryo | suspicious, follow-up scan advised |
| No embryo with a heartbeat 2 weeks after a scan showing a sac without a yolk sac | diagnostic of pregnancy loss |
Suspicious findings call for a follow-up scan, usually 7 to 14 days later, rather than a decision based on one scan.
Example
A sac measuring 10, 9 and 11 mm has an MSD of 10 mm, which points to a gestational age of about 5 weeks and 5 days on the day of the scan.
Limits
Sac measurements vary with the operator and with the shape of the sac. Use this calculator to understand a report, not to make decisions; your care team interprets the scan with your dates, symptoms and hCG levels.