One twin measuring behind?

I had my first ultrasound at a perinatal clinic today (my 4th total). According to my due date I am 10w2d. Baby A is measuring 9w6d, and they aren't really worried about it. Baby B is measuring at 9w2d but has consistently measured almost exactly a week behind my due date at every ultrasound. I spoke with a doctor today briefly who said he couldn't see anything wrong with Baby B as of right now because it was so early but was throwing around terms like "size discrepancy" and "possible birth defects" which only worried us... I guess I'm mostly wondering if other moms have had a similar experience with size difference in twins and if your babies showed any "problems" in the future? Thanks :)