Size discrepancy

Hello! I am 12.5 weeks pregnant with mo/di twins. Since the first ultrasound at 7 weeks Baby A has been consistently measuring a few days behind due date and Baby B is measuring a few days ahead. I've had three ultrasounds and it has been consistently like this. I realize there is a risk of TTTS with mo/di twins but the doctors seem to think this early and consistent difference is related to the embryo splitting unevenly and they may always be slightly different sizes. Anyone else have this with their identical twins?