20 C.F.R. § 404.357
You may be eligible for benefits as the insured's stepchild if, after your birth, your natural or adopting parent married the insured. You also may be eligible as a stepchild if you were conceived prior to the marriage of your natural parent to the insured but were born after the marriage and the insured is not your natural parent. The marriage between the insured and your parent must be a valid marriage under State law or a marriage which would be valid except for a legal impediment described in § 404.346(a) . If the insured is alive when you apply, you must have been his or her stepchild for at least 1 year immediately preceding the day you apply. For purposes of determining whether the conditions of entitlement are met throughout the first month as stated in § 404.352(a)(2)(i) , you will be considered to meet the one year duration requirement throughout the month in which the anniversary of the marriage occurs. If the insured is not alive when you apply, you must have been his or her stepchild for at least 9 months immediately preceding the day the insured died. This 9-month requirement will not have to be met if the marriage between the insured and your parent lasted less than 9 months under one of the conditions described in § 404.335(a)(2)(i)-(iii) .
20 C.F.R. §404.357