Gestational Sac and Its Meaning in Pregnancy

Presence of the gestational sac on transvaginal ultrasound

Medical illustration of fetus development at 9 weeks inside the gestational sac
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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 1500 and 2000.

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.

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 1500 if you are not as far along in your pregnancy as you thought).
  • You have an ectopic pregnancy. If your hCG levels are between 1500 and 2000 but a gestational sac is not seen, it can indicate an ectopic pregnancy.
  • 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?

An embryo is usually seen within the gestational sac by 6 weeks gestation. One of the more common types of miscarriages—known as an an embryonic 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 woman even realizes she was pregnant. It can be the result of abnormal cell division, poor quality sperm, or a poor quality egg. 

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

This procedure may be desirable for women 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 her physician. 

What If My Doctor Sees an Empty Gestational Sac?

If your doctor discovers an empty gestational sac on an ultrasound, he 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.

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  2. Puget C, Joueidi Y, Bauville E, et al. Serial hCG and progesterone levels to predict early pregnancy outcomes in pregnancies of uncertain viability: A prospective studyEur J Obstet Gynecol Reprod Biol. 2018;220:100-105. doi:10.1016/j.ejogrb.2017.11.020

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