The first part of the pregnancy that can be seen is the gestational sac. The gestational sac can be seen around cycle days 32-35 (4w+3d-5w+0d), about 18-21 days after ovulation/fertilization, though it sometimes can take a couple of days longer to see the gestational sac on transvaginal ultrasound.
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.