The Meaning of Small Gestational Sac in Early Pregnancy

Pregnant woman having sonogram

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The gestational sac is the fluid-filled structure that surrounds the embryo in the womb. With a transvaginal ultrasound (an exam during which high-frequency sound waves produce an image), the sac can be seen very early in pregnancy—usually around four to five weeks after your last menstrual period, when its diameter is only about 2 to 3 millimeters.

Sometimes 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. A small gestational sac may mean nothing, or it may be a cause for concern. Getting a series of ultrasound exams as your pregnancy progresses will help your doctor interpret what, exactly, it means.

An Error in Timing

In very early pregnancy, especially during a first ultrasound, a smaller-than-expected gestational sac could simply mean that the pregnancy is earlier along than you expected based on the date of your last menstrual period. This is rather common, given that many women do not have predictable 28-day menstrual cycles with ovulation occurring exactly in the middle of the 14th day.

A menstrual cycle might be anywhere from 21 to 35 days long, and ovulation doesn't always occur on day 14 of the cycle, regardless of length of cycle. It's also possible that you accidentally misremembered the date of your last menstrual period. This is easy to do, especially if you weren't expecting to get pregnant and weren't paying close attention to your cycle.

In this situation, the next step is to schedule a follow-up ultrasound at whatever point in time in the future that your doctor recommends. During that second exam, your doctor will measure the size of your gestational sac again.

If the pregnancy is progressing normally, the sac should be growing appropriately. If it is, your doctor may then revise the estimated due date based on other findings within the ultrasound examination.

A Warning Sign of Pregnancy Loss

In other cases, unfortunately, a small gestational sac can be concerning. It may sometimes—but not always—be a warning sign of impending pregnancy loss when follow-up ultrasounds continue to show a small sac size. In these cases, your doctor will probably recommend continued monitoring until there is enough information to determine whether or not the pregnancy is viable.

Your doctor is likely to use other tools and tests beyond simply examining the size of the gestational sac to determine whether or not your pregnancy is healthy. For instance, they are likely to test your level of human chorionic gonadotropin (hCG), a hormone that your body produces when pregnant. If your hCG level isn't doubling every two to three days or is dropping, those can also be a sign of impending pregnancy loss.

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