Gestational Sac: 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 amniotic fluid (five to seven weeks after the last menstrual period in natural cycles).

The sac is found in the uterus, even in cases of ectopic pregnancy. On an ultrasound, it appears as a white rim around a clear center.

This sac is usually visible between three and five weeks gestational age using a transvaginal ultrasound, which has higher sensitivity and produces clearer images than a transabdominal ultrasound. A transvaginal ultrasound can help identifying any abnormalities, including a miscarriage or ectopic pregnancy, as well as date a pregnancy more accurately than other methods. 

Signs of Trouble

Although a gestational sac visible on ultrasound is a positive sign of pregnancy, it does not guarantee the pregnancy or baby 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. In other cases, an ultrasound may demonstrate that the gestational sac is abnormally shaped or not increasing as it should, by about 1 mm per day in early pregnancy. In yet other cases, the sac is abnormally shaped.

Normal gestational sacs are round or oval and located within the mid-portion, or fundus, of the uterus.  

Empty Gestational Sacs

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. This type of pregnancy failure in which the 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.


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