Please Tell Me this could end well...

I'm having twins. One twin is measuring a week behind the other. They both have strong heart beats but my doctor keeps telling me I could still miscarry the smaller twin. He had continued to develop farther at each appointment. Just isn't at the same place as baby a. Please tell me there is still a chance that I'll have both babies! I'm freaking out because my doctor is worried about it still. I'm 11 weeks and days.