Ultrasounds this early are iffy because the embryo is moving so fast at this point in development. One minute it is hardly showing and the next minute it is there. If your doctors say all you can do is check again, they're the best judges of the situation. I know it is hard to wait, but it might be just fine. Only time will tell.
I got pregnant with the help of an IVF clinic whose clientele largely pays their hefty fees out of pocket (since most insurance companies don't cover IVF). I'm sure it would make them a ton of money to do ultrasounds for their eager patients at 4 weeks, 5 weeks, etc. (You bet I would have done it if they would have let me.) But even this place, with every financial incentive in the world to do ultrasounds whenever, refuses to do them until week 7. This is for the very reason that you have run into -- a week-6 ultrasound is equivocal. (Sure, if they see a yolk, that's great, but if they didn't see a yolk, what does that mean? Is it just too early? Something wrong?) Then they would have to check again with another ultrasound anyway, and especially, it distresses the patient for an extra week or two, so often unnecessarily.
Don't drive yourself crazy trying to second-guess the situation; many ladies do get totally normal readings at 8- and 9-week ultrasounds. I'll cross my fingers that you do too.
With my last pregnancy at age 37, I had a 6 week ultrasound also which showed ONLY an empty sac. No yolk, no fetal pole. no baby. Same boat as you.I spent two weeks in agony not knowing what to expect.
But at 8 weeks, everything was there, and that "non-existant" baby is now a darling, healthy, happy 17 month old boy.
One thing I was told that made so much sense is that---just as babies and children differ in growth rates--so can a fetus and/or embryo.So it could be totally normal for YOUR baby to not show up at 6 weeks and still be just fine.
We really do hear lots of these stories on here and many go on to be just fine like I was.
My fingers are crossed for you! I know exactly what you are going through!