One baby is almost 1 week smaller!!!

I'm 11w1d with Di-Di twins. Has anyone had this happen? If so, how did it turn out? Baby B has always been 2-4 days smaller than Baby A with a slightly slower heart rate, slightly less active and slightly lower heart rate. Today Baby A measured right on track at 11w1d but Baby B measured at between 10w2d -10w3d (so 5-6d smaller).....B's sack is also smaller. Heart rates were good on both, both were moving around and looked equally developed (arms & legs etc) except for the RE said that it's possible Baby B won't make it or that it will be fine and I have to wait and see. I'm following up with my OB (waiting for a call).....but now I'm super worried!!! I saw a perinatalogist last week and did a brief scan and bloodwork for MaterniT21 test (no results yet). They didn't tell me anything about their growth worried!!! 😥