Gestational Sac and Its Meaning in Pregnancy

Presence of the gestational sac on transvaginal ultrasound

Photo illustration of pregnant belly

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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. What exactly is the gestational sac? When can it first be detected on ultrasound? What does it mean if it is (or is not) there? What does it mean 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.

The gestational sac forms around five to seven weeks after the last menstrual period in natural cycles, so it is usually visible between 3 and 5 weeks gestational age using a transvaginal ultrasound.

Transvaginal ultrasound has a higher sensitivity and produces clearer images than a transabdominal ultrasound. The gestational sac can usually be seen once your hCG levels are between 1,500 and 2,000 mIU/mL. However, one 2013 study found that hCG levels that correspond to detection of the gestational sac can vary widely, from 390 to 3,510 mIU/mL.

When a Gestational Sac Is Seen on the Ultrasound

Visualizing a gestational sac is a positive sign of pregnancy, but it is not a guarantee that your pregnancy is healthy and will proceed normally. 

After the sac 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?

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).

An ectopic pregnancy can be a medical emergency, and if this is a possibility your doctor will want to do further testing and talk about treatment options.

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 their 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 If My Doctor Sees an Empty Gestational Sac?

If your doctor 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). 

A Word From Verywell

Early transvaginal ultrasounds are a relatively easy way to follow a pregnancy early on. Paired with your hCG levels, ultrasounds can give you and your doctor an idea about how your pregnancy is progressing.

The gestational sac is the first structure your doctor 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 (without evidence of an embryo by 6 weeks gestation).

In some cases, a gestational sac is not seen at all. The most common reason for this is inaccurate dates or it is simply too soon. If a gestational sac is not seen on a follow-up scan, or if your hCG levels indicate one should be seen, it can be a sign of a miscarriage or ectopic pregnancy.

Pregnancy can be a joyful time, but you it can also be anxiety-ridden if it's thought that you are having a miscarriage, have a blighted ovum, or an ectopic pregnancy. If you have not yet shared your pregnancy with family and friends, you may feel very alone. Lean on your friends and family members during this time if you can.

People in your network of support might offer well-intended but hurtful comments such as "you can always have another," but you need to honor your feelings. There are stages of grief associated with miscarriage and it's important that you grieve in the way that feels right to you.

10 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading
  • Cunningham, F. Gary, and John Whitridge Williams. Williams Obstetrics. New York: McGraw-Hill Education Medical, 2014.

By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.