My lmp was 1/8/12, I ovulated on 1/22/12 (use an ovulation test) and had a positive hpt on 2/6/12. I should be 7 weeks, but when we went to the OB, he did a sonogram and said I was 5 weeks. He wants me to come next week to see how the baby is progressing. Should I be concerned? We didn't see anything but a round dark space, but no heartbeat or anything.

Not necessarily as dates can be off. Ultrasound won't be able to date exactly, but early on it is pretty close as babies tend to develop in a pretty uniform manner. In the early weeks, a matter of days can make a difference in what is seen on a scan. I know it is frustrating, and causes some anxiety with not being sure, but see what happens when you have the next scan. Hopefully you will gain some peace of mind at your next visit.