Gestational sac signs on USG

Sonographic Appearances of Normal Intrauterine G-sac:
It should be possible to visualize a normal G-sac via TVS between 4.5 to 5 weeks of MGA.
The threshold level identifies the earliest we can visualize the sac :- 4 weeks + 3 days.
The discriminatory level indicates when we should always see the sac :- 5 weeks + 2 days.
The earliest appearance of gestation on TVS is referred to as Intra Decidual Sign. It is seen as a small gestational sac within within the decidua.

 Diagram of the intradecidual sign. The gestational sac (circle) does not displace or deform the central cavity complex (straight black line). The white area represents thickened decidual tissue.

This is followed by the Double Decidual sign. It is seen as two distinct echogenic layers out of which the inner ring denotes the G-sac while the outer ring denotes the echogenic endometrial lining. This sign is helpful in differentiating it from a fluid filled decidual cast of an ectopic pregnancy.

Diagram of the double decidual sac sign. The gestational sac (circle) protrudes into and displaces the central cavity echo complex (curved line). The white area represents thickened decidual tissue.