37 CFR 1.451 - The priority claim and priority document in an international application.
(b) Whenever the priority of an earlier United States national application or international application filed with the United States Receiving Office is claimed in an international application, the applicant may request in the Request or in a letter of transmittal accompanying the international application upon filing with the United States Receiving Office or in a separate letter filed in the United States Receiving Office not later than 16 months after the priority date, that the United States Patent and Trademark Office prepare a certified copy of the prior application for transmittal to the International Bureau (PCT Article 8 and PCT Rule 17). The fee for preparing a certified copy is set forth in § 1.19(b)(1).
(c) If a certified copy of the priority document is not submitted together with the international application on filing, or, if the priority application was filed in the United States and a request and appropriate payment for preparation of such a certified copy do not accompany the international application on filing or are not filed within 16 months of the priority date, the certified copy of the priority document must be furnished by the applicant to the International Bureau or to the United States Receiving Office within the time limit specified in PCT Rule 17.1(a).
(d) The applicant may correct or add a priority claim in accordance with PCT Rule 26bis.1.