Goals of First Trimester Sonography:

Visualisation of G-sac.

Localisation of G-sac.

Detection of early pregnancy failure.

Determining number of embryos and chorionicity.

Estimation of Gestational age.

Early diagnosis of fetal anomalies.

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.

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.