Gestational Sac

Definition - What does Gestational Sac mean?

The gestational sac is the fluid-filled structure that encloses a developing embryo in the very early stages of pregnancy. It is the first visual evidence of pregnancy, identified via ultrasound in the initial 5-7 weeks of gestation before the embryo is even visible. Starting as early as 5 weeks, gestational age can be determined based on the diameter of the gestational sac.

The presence of a gestational sac may indicate whether or not a healthy pregnancy is developing and indicate if any issues are present.

FertilitySmarts explains Gestational Sac

The gestational sac appears on an ultrasound as a dark sphere surrounded by a thin, white ring. The sac will typically be located within the upper part of the uterus called the fundus.

Inside of the gestational sac, a smaller sac called the yolk sac should appear. The yolk sac is responsible for the nourishment of the embryo before the placenta forms.

Several situations may indicate concerns regarding an early pregnancy:

  • Empty gestational sac. This occurs when an embryo implants but fails to survive. In this case, the body responds to early pregnancy triggers and the gestational sac is developed, however, there is no longer a fetus growing inside. This early type of miscarriage often goes unnoticed by the woman, as the pregnancy would have existed for such a short period of time.
  • Gestational sac fails to grow, or yolk sac fails to develop. Either condition indicates that the pregnancy is not progressing.
  • No gestational sac is visible. Following a positive pregnancy test, a lack of gestational sac may indicate an ectopic pregnancy.


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