Will a mammogram compress fat after breast fat transfer? Should I be concerned about possible shape alteration or necrosis?

The answer to your question should be answered by your plastic surgeon.
Each provider will have their own view on what’s appropriate and what’s not appropriate.
Grafted the tissues establish a new blood supply within the first week or the tissue will die.
That blood supply is gradually strengthened through increased neovascularization and the formation of scar tissue which holds tissue in place.
The most sensitive time for graft a tissue is within the first few weeks.
This is when grafted a tissue is most susceptible to not reestablishing a blood supply and becoming in the chronic.
At four weeks whatever fat is alive has a blood supply and there is no need for further differentiation.
The question is if the new blood supply is still fragile to pressure and could be damaged from an ultrasound.
Four weeks is probably relatively safe but I think most plastic surgeons would probably wait a little longer before putting grade pressure on your breast.
At three months it’s highly unlikely any external force will have any impact on the outcome of the procedure.
Results are typically final by three months and patients should not expect much change after that point.
Opinions will vary in regards to the timing of your ultrasound and it is probably inappropriate for us to give you medical recommendations without an established doctor patient relationship.
Follow up with your plastic surgeon. He or she are responsible for the outcome of your procedure and can give you the best advice.
Best,
Mats Hagstrom MD