Publication of a non-provisional patent application by the U.S. Patent Office occurs 18 months from its earliest effective priority date. See MPEP 1120. The earliest effective priority date is the earlier of the patent application’s filing date or the filing date of an earlier patent application that a claim of priority is properly made. If the utility patent application claims the filing date benefit of a provisional patent application, then the 18 month period is calculated from the provisional patent application’s filing date. Please note, only the non-provisional patent application is published and not provisional patent applications nor design patent applications. (Provisional patent applications are only published in a rare circumstances.
When a U.S. patent application is published, the entire originally filed patent application is made publicly available for download and entire file history for the patent application can be viewed by anyone using Public PAIR. (see http://portal.uspto.gov/pair/PublicPair ) Not only can any third-party see your original patent application they can also see all the documents later filed by your (ie. Information Disclosure Statements, etc), all Office Actions sent by the U.S. Patent Office and responses/Amendments filed with the U.S. Patent Office. In addition, your published patent application becomes prior art for any later filed patent application by others or by you.
Publication of a U.S. patent application is required by the U.S. Patent Office if you (i) have filed or (ii) intend to file in a foreign country an international patent application for the invention. For example, if you filed a PCT international patent application 30 days prior to your U.S. patent application, the U.S. patent application will have to be published within 18 months of the earliest effective priority date.