Sometimes the ultrasound measurements will reveal a gestational sac that is smaller than expected. But it can be difficult to draw conclusions based on a single early ultrasound.
The significance of a small gestational sac can vary from one situation to another.
In very early pregnancy, especially on a first ultrasound, a smaller than expected gestational sac could mean that the pregnancy is simply earlier along than was expected based on the last menstrual period date. This is rather common, given that many women do not have predictable 28-day menstrual cycles with ovulation on the 14th day.
If the explanation for a small gestational sac is that a pregnancy is earlier than expected, a followup ultrasound should show a gestational sac with appropriate growth. The doctor may then revise the estimated due date based on the ultrasound results.
In other cases, a small gestational sac can be a cause for concern, and it may sometimes (but not always) be a warning sign for pregnancy loss when followup ultrasounds continue to show a small sac size in comparison to other ultrasound measurements. In these cases, the doctor will probably recommend continued monitoring until there is enough information to determine whether or not the pregnancy is viable.
Bromley, B, BL Harlow, LA Laboda and BR Benacerraf. "Small sac size in the first trimester: A predictor of poor fetal outcome." Radiology 1991. Vol 178, 375-377.
Oh, Dr J. S., G. Wright, C. B. Coulam. "Gestational sac diameter in very early pregnancy as a predictor of fetal outcome ." Ultrasound in Obstetrics and Gynecology Dec 2002. 3:267 - 269.
Ultrasound. American Pregnancy Association. Accessed: Jan 7, 2009. http://www.americanpregnancy.org/prenataltesting/ultrasound.html