The gestational sac is the extraembryonic sac or the sac present initially in Blastocyst?

The gestational sac is the extraembryonic sac or the sac present initially in Blastocyst?

Gestational sac is the first sac and also the first sign on USG for pregnancy.

Ayesha what are you trying to ask ??

maam the Gestational sac is which part of the developing embryo. Is it the empty fluid filled cavity that is initially formed in Blastocyst?