Pregnancy associated breast cancer

pregnancy associated breast cancer also includes cancer within 1 year of delivery. So during this period why is the IoC USG and not others like MRI or mammogram(not good in young woman being pregnant) as the baby is delivered ?

MRI can be done

The hormonal effects of estrogen and progesterone still persist in this period.

IOC is hence USG.

Okay thanks. But is there any reason for preferring USG over MRI?