Gestational Sac

The gestational sac is a fluid-filled structure that appears early in pregnancy and surrounds the developing embryo and yolk sac at approximately 4.5- 5 weeks gestational age. It's often described as a "fluid-filled ring" on ultrasound , Surrounds and contains the fetal pole and yolk sac and is visible. The gestational sac forms within the uterine lining (endometrium) shortly after implantation of the fertilized egg.

Its size increases steadily throughout the first trimester, providing a protective environment for the growing embryo. The mean sac diameter (MSD) is measured during early pregnancy ultrasounds.

Features of Concern :The absence of a heartbeat (cardiac activity) within the embryo, No embryo identified with MSD between 16 and 24 mm, Large yolk sac (>7 mm) is a definitive sign of embryonic demise.

Delayed identification of embryo with cardiac activity in follow-up scans. Abnormally slow embryonic heart rate (bradycardia). Disproportionately small sac relative to embryo .Distorted contour, thin/irregular rim, low position. Abnormally large amniotic sac compared to embryo , Abnormal yolk sac appearance (calcified, etc.),Double bleb sign (5-7 weeks, increased risk of loss).