Could identical twins have different gestational ages on ultrsound?

Dear Ask The Doctor: Could identical twins have different gestational ages on ultrsound? about 1 week to ten days difference....? keeping in mind that my twins have each a separate sac.... but i'm not sure if they have a different placenta... since in week 17 the ultrasound showed to be one placenta... but i have a feeling it was two placenta and it became close and showed one.... is it possible?

Dear Dina: No. Identical twins would be conceived at the same time but one twin could be growing at a different rate as the other twin thus accounting for the difference in size.