Most practitioners wait until at least 6 weeks to perform the first pregnancy ultrasound. However, a gestational sac can be seen as early as 4 1/2 weeks after your last period, and a fetal heartbeat can be detected at 5 to 6 weeks (though that isnt always the case).

Can you see anything on an ultrasound at 4 weeks?

The gestational sac contains amniotic fluid and surrounds the embryo. You may see the gestational sac in an ultrasound as early as 4 1/2 to 5 weeks.