If a State plan under this subchapter provides for payments for inpatient hospital services on a prospective basis (whether per diem, per case, or otherwise), in order for the plan to be considered to have met such requirement of
section 1396a(a)(13)(A) of this title as of
July 1, 1989, the State must submit to the Secretary by not later than
April 1, 1989, a State plan amendment that provides, in the case of hospitals defined by the State as disproportionate share hospitals under paragraph (1)(A), for an outlier adjustment in payment amounts for medically necessary inpatient hospital services provided on or after
July 1, 1989, involving exceptionally high costs or exceptionally long lengths of stay for individuals under one year of age.