As 2 of the other ladies said, at 5 weeks you can see the gestational sac, at 6 weeks you can see the fetal pole (baby) and possibly a heartbeat.
You can see baby at 6 weeks, including heartbeat. Most dr's won't do an ultrasound before that because all you see is the sac and no heart beat so it kind of freaks mom's out .... But the baby's heart beat can be seen between 6-8 weeks.
i would say 8-10 week sonogram because the baby is real small at that age
At 5 weeks pregnant (from last period) you would be able to see the gestational sac, and by 6 weeks you should be able to see the babies heart beat.