Gestational Sac and Its Meaning in Pregnancy

After a home pregnancy test has turned positive and a blood test measuring levels of human chorionic gonadotropin (hCG) have confirmed pregnancy, the next proof of pregnancy requires an ultrasound.

When you have your first ultrasound, your doctor may talk about the presence or absence of the gestational sac...but it's okay if you have some questions about what, exactly, that is. You might be wondering when the gestational sac can first be detected, and what it means if it isn't there. OR what happens if a gestational sac is seen, but appears to be empty?

Here are the answers to these, and other, common questions you might have about the role of the gestational sac in your pregnancy.

What Is the Gestational Sac?

One of the first signs of pregnancy on an ultrasound is the gestational sac, which encloses the developing baby and contains amniotic fluid. The gestational sac is found in the uterus. On the ultrasound image, it appears as a white rim around a clear center.

When Is the Gestational Sac Seen on the Ultrasound?

After the gestational sac sac first becomes visible, the next positive sign of pregnancy is a yolk sac that develops within it. The yolk sac provides nutrition to the developing embryo until the placenta takes over. It's an important indicator of pregnancy health.

In some cases, a gestational sac will be detected on ultrasound, but a yolk sac is subsequently not found. The yolk sac usually becomes visible on a transvaginal ultrasound between 5 1/2 and 6 weeks gestation.

Along with the gestational and yolk sacs, the fetal pole (the earliest stage of the embryo) is also visible around 6 weeks pregnant. This appears as a linear or oval-shaped structure that sits adjacent to the yolk sac, measuring around 1 to 2 millimeters. At this point it is featureless, but begins to take shape around 7 to 8 weeks when the spine begins to form.

What If a Gestational Sac Is Not Seen on an Ultrasound?

If a gestational sac is not seen on your ultrasound, what does that mean? There are several possible reasons for a lack of a gestational sac. It could be that:

• Your dates are off. Miscalculated dates are a common reason a gestational sac is not seen and can simply mean that you need a repeat ultrasound later on. Your doctor may compare the finding with your hCG levels (which would likely be lower than expected if you are not as far along in your pregnancy as you thought).
• You have an ectopic pregnancy. If your hCG levels are normal but a gestational sac is not seen, it may indicate an ectopic pregnancy. However, the American College of Obstetrics and Gynecology notes that consecutive hCG measurements, taken two to seven days apart to track changes, are necessary for an accurate diagnosis.
• You have miscarried. If the gestational sac is not seen and your hCG levels are falling, it can indicate an early miscarriage (chemical pregnancy).

What Does an Empty Gestational Sac Mean?

A yolk sac, which indicates a viable pregnancy, is usually seen within the gestational sac by 35 days gestation. One of the more common types of miscarriages—known as an anembryonic pregnancy, empty sac, or blighted ovum—happens when a gestational sac does not contain an embryo. In other words, an embryo failed to develop.

This type of pregnancy loss occurs early in the first trimester, and often before a person even realizes they are pregnant. Risk factors for anembryonic pregnancy include:

• Abnormal cell division
• Chromosomal abnormalities in the fertilized egg
• Infection, an autoimmune disease, or endocrine disorders in the pregnant person

In most cases, chromosomal abnormalities will cause the person's body to miscarry naturally and without intervention. However, there are some cases in which a dilation and curettage (D&C) is necessary to complete the miscarriage.

This procedure may be desirable for people who want a pathologist to try to find a cause for the miscarriage, for those who feel it will help them cope better with the loss, or for physical or medical concerns raised by a physician. There is also an option for medical management of a miscarriage which involves taking a medication to help your body expel the failed pregnancy.

What Happens If My Health Care Provider Sees an Empty Gestational Sac?

If your provider discovers an empty gestational sac on an ultrasound, they may confirm that your pregnancy is nonviable—in other words, that the pregnancy will not result in the birth of a baby as it is not progressing normally.

Depending on the size of the gestational sac, it might just be too early to determine that the sac is truly "empty." In this case, your doctor will ask you to return for a repeat ultrasound to ensure an accurate diagnosis (which means determining that the pregnancy is either viable or nonviable). 

The Bottom Line

The gestational sac is the first structure a health care prvovider will look for with an early ultrasound. When it is present (between 3 and 5 weeks gestation), it can be a positive sign. Sometimes, a gestational sac is seen but is found to be empty, and in other cases, a gestational sac is not seen at all.

If you're experiencing pregnancy loss, finding a support system, whether it's friends, family members, a partner, or even a group of others in similar situations, can be beneficial. Talk with a medical professional about your options for mental health care, and remember that you aren't alone.