Vanishing twin or is baby behind?

Went in for an early ultrasound as I have a history of miscarriages. I was seven weeks two days. We saw two gestational sacs, and both had a yolk sac. but one had a baby measuring seven weeks and three days with heart rate of 152. The other one had just the yolk sac. Is this a vanishing twin or is it possible the other baby is just behind? Could they be that far behind and still catch up? They said the empty gestational sac was measuring six weeks five days, so not that far behind. But again I didn’t see a baby, only a yolk sac... I go back next week for a repeat ultrasound. But the midwife thought it was a vanishing twin.