The<unk>  is the first element seen in the gestational<unk>  during pregnancy, usually at 5 weeks gestation.
It is a critical landmark, identifying a true gestation<unk> .
It is quite echogenic (light) to ultrasound, and reliably seen early.
Yolk sac
Amniotic sac
Mesonephros
Fallopian tube
Spermatic cord
Callus
Notochord
Metanephric blastema