MRI and US in the assessment of placental health and disease

MR and ultrasound are listed in separate categories. However, US will always be a screening tool, used prior to MRI. When we are assessing diseases with plancetal etiologies, what is the best manner to integrate the information provided by both US and MRI. Can ultrasound be used to screen to lmiit the number of MR examinations? How is this affected by gestational age, position of the placenta with respect to maternal body habitus, and patient risk factors