Gestational Sac and Its Meaning in Pregnancy

One of the Earliest Signs of Pregnancy

Gestational Sac
Image of a Gestational Sac in Early Pregnancy. Credit: Photo © A.D.A.M.

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

One of the first signs of pregnancy to show up on ultrasound is the gestational sac, which encloses the developing baby and contains amniotic fluid

The gestational sac is found in the uterus and on an ultrasound, it appears as a white rim around a clear center.

This sac forms around five to seven weeks after the last menstrual period in natural cycles, so it is usually visible between three and five weeks gestational age using a transvaginal ultrasound—this type of ultrasound has a higher sensitivity and produces clearer images than a transabdominal ultrasound.

Is A Gestational Sac A Guarantee Of A Normal Pregnancy?

Although a gestational sac visible on ultrasound is a positive sign of pregnancy, it does not guarantee the pregnancy is healthy and will proceed normally. For example, 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, and is thus 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. 

What Does An Empty Gestational Sac Mean?

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 woman even realizes she was pregnant. It can be the result of abnormal cell division, poor quality sperm, or 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 carry out 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 others words, that the pregnancy will not result in the birth of a baby, as it is not progressing normally.

But sometimes (depending on the size of the gestational sac), it may ​be a bit too early to determine that the sac is truly "empty." In this instance, your doctor will ask you to return for a repeat ultrasound. This can be an anxiety-ridden time but is meant to ensure a 100 percent accurate diagnosis (that the pregnancy is either viable or nonviable).  

Source:

American College of Obstetrics and Gynecology. (May 2015). Practice Bulletin: Early Pregnancy Loss.

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