Me and My wife just had our first U/S and they only saw a sack. no yolk no embryo.  They did say the sack looked great nice and round.  
She is due to have another scan in 2 weeks. The Dr said she was probably only 5 weeks not 7. We used an ovulation stick so we know when we tried. 
My wife has been pretty upset since. We have been trying to have children for some time.  
I can say the U/S looked awful compared to all the pictures I see online. the ones online have definitive black circles. Our was just lighter or darker shades of grey. They said they could not see anything vaginally and could only go from the belly. My wife is also plus size not sure if that makes it harder to see anything. Was everyone else's just as hard to see? We could see the uterus and then the sack inside.
I know just to wait, but everything i read they at least see the yolk. The next two weeks are going to be hard. I guess we can only wait and see.
Re: First U/S slightly worried - week 7
They lied to you. I didn't have an abdominal u/s until 10+ weeks all previous ones, 6,7, and 8 weeks were all vaginal and we saw everything perfectly clear.
This as well. Vaginal ultrasounds are standard before 10 weeks with all the doctors I have seen.
The plus size could contribute to why they couldn't see that much that way also. I say hang in there and don't stress, good luck
We have just had out first U/S @ 6w3d due to a threatened miscarriage.
The tech started with an abdominal scan and like you, we saw the sac and not much else.
Luckily we had a trainee tech who wanted to do everything properly (she was an amazing for a trainee!!) and decided to do a vaginal to give us more 'definitive answers'. With the vaginal we saw our baby clear as day, with a strong heartbeat.
So, don't be too discouraged as not only could your wife still be early on, but also the abdominal doesn't show much at all at this point.
Push for a vaginal next appointment if it is not offered.
Good luck!!
At this point it's very early and so the baby is small. I had a 6 week u/s and we saw the baby pretty quickly. Oddly, at the 7 week u/s we could not find it for a very, very long time. In fact, we'd gotten to the point where they were saying that I would have to come back in a week but I wanted them to keep looking. They looked for a lot longer than usual and we ended up locating the baby. It was hiding out below and it was just really hard to see, until we found it. The nurse said this can happen and is normal and nothing to worry about.
My u/s was a vaginal u/s. I don't think that you would be able to see anything externally, being plus sized (as I am), at this point. I know it's so hard not to worry but tell her to try to relax and that this does happen often in a viable pregnancy. It's just so early and if you did an external u/s then that's making it even more difficult.