42 CFR 435.915 – Effective date
(a) The agency must make eligibility for Medicaid effective no later than the third month before the month of application if the individual—
(1) Received Medicaid services, at any time during that period, of a type covered under the plan; and
(2) Would have been eligible for Medicaid at the time he received the services if he had applied (or someone had applied for him), regardless of whether the individual is alive when application for Medicaid is made.
(b) The agency may make eligiblity for Medicaid effective on the first day of a month if an individual was eligible at any time during that month.
(c) The State plan must specify the date on which eligibility will be made effective.