On an abdominal ultrasound, the sac can be seen several days later.
The first that you can usually see on a transvaginal ultrasound (around 5+ weeks) is the gestational sac (GA). The gestational sac is a structure that surrounds the embryo and can be seen even before the embryo is visible. About 7-10 days later the embryo can be seen on transvaginal ultrasound at around 6w3d plus the fetal heart beat.
You cannot usually see the pregnancy on a transvaginal ultrasound until a blood hCG level of at least 1,500.