5.5 weeks no baby?

I had an ultrasound yesterday and I'm measuring 5.5. There was a yolk sack but no fetal pole yet? My OB said she is guessing blighted ovum? I thought a fetal pole until 6weeks? I had my beta drawn and go back for a repeat Saturday. My hpt's have all been getting nicely dark. Should I worry or should I be preparing? Is my OB correct with what she said? I have a repeat ultrasound on the 21'st. :0(