Gestational sac
First visible sign of pregnancy on ultrasound, appearing as a fluid-filled structure containing the developing embryo.
Full Definition
The gestational sac is the earliest sonographic evidence of pregnancy, visible as early as 4-5 weeks gestational age as a small, round, anechoic (fluid-filled) structure within the uterus. It represents the chorionic cavity surrounded by trophoblastic tissue and should demonstrate a clear interface with surrounding endometrium. The mean sac diameter is used for early gestational age estimation before the embryo becomes visible. A gestational sac without a visible fetal pole after 6-7 weeks may indicate embryonic demise or anembryonic pregnancy.
Usage
Usage note: Always written as two words. Distinguish from yolk sac, which appears later and is smaller.
In Context
- "A well-defined gestational sac measuring 12 mm was visualized in the fundal region at 5 weeks 3 days." — Early pregnancy ultrasound report
- "The absence of a fetal pole within the gestational sac at 7 weeks suggests possible embryonic demise." — Clinical assessment