The gestational sac is the earliest sonographic evidence of pregnancy, typically visible on transvaginal ultrasound by about 4.5 to 5 weeks of gestational age. It appears as a small, round or oval, anechoic fluid collection within the decidualized endometrium, surrounded by a bright echogenic rim. As pregnancy progresses, a yolk sac appears within the gestational sac, followed by the fetal pole with cardiac activity. The mean sac diameter is measured to help estimate early gestational age. An abnormally shaped, small, or low-positioned gestational sac, or one that grows more slowly than expected, can suggest an abnormal pregnancy. Distinguishing a true gestational sac from a pseudogestational sac in ectopic pregnancy is an important early diagnostic challenge.
Sonography Term