Measuring 5 wks 5 days gestational sac, no yolk sac.

I went for an ultrasound yesterday. According to my periods, I'm supposed to be 5 wks 2 days pregnant. But yesterday I had an ultrasound and they measured the gestational sac at 5 wks 5 days...but no yolk sac. My hormone levels are at 4200 3 days ago so they are optiomistic that it's just a slow moving pregnancy. They could see thickening around the sac, no fluid in my cervix and high levels of hcg so they aren't giving it much concern...but I am. According to my last ultrasounds with my other children (I'm a mother of 3 girls) they found a yolk sac at about 5 weeks. I've had a blightned ovum before, about 10 years ago, and my levels kept rising until I started bleeding about 8 weeks. At 10 weeks they did an u/s and found a sac, but no yolk sac and I miscarried about 2 weeks later on Christmas day. I figured by now the least I'd see was a yolk sac...but I'm just seeing a gestational sac. I'm confused. Anyone else have the same problem where the gestational sac is measuring more than what it should and no yolk sac and end up to have perfectly normal pregnancy??