When Can the Gestational Sac Be Seen?

The gestational sac is the first anatomical structure visible on ultrasound that confirms an intrauterine pregnancy. The gestational sac surrounds and protects the embryo during the early weeks of pregnancy. The sac consists of two layers: the inner layer – the amnion – which contains the amniotic fluid, and the outer layer – the chorion – which will form the placenta.

The gestational sac develops shortly after the embryo´s implantation in the uterus and is usually visible by week 5 of pregnancy. However, waiting an additional week is recommended for a more accurate diagnosis.

On ultrasound examination, the gestational sac appears as a dark, echo-free area surrounded by a bright, echogenic halo within the endometrium.

The presence of the gestational sac in the uterus confirms that the pregnancy is not ectopic (that is, implanted outside the uterus). Additionally, its size and shape provide information about the pregnancy´s progression.