You may not enter the United States to take up residence ahead of the U.S. Citizen petitioner.
You are required to present the immigrant visa to the immigration officer at your port of entry into the United States, at which time, you will be given the forms you are required to complete to apply for a Permanent Resident Card (PRC), commonly known as a “Green Card”. The card will be mailed to the address of your final destination in the United States at a later date. Your passport will be stamped to show that you have been admitted as a Lawful Permanent Resident or Conditional Resident and that an application for an PRC is being processed.
What if I want to leave the United States?
The stamped visa in your passport acts as a temporary PRC for up to 12 months. If you leave the United States before the PRC is recevied, you may re-enter the country using the stamp in your passport, provided it is still valid. Should the stamp expire while you are abroad, you should contact the U.S. Embassy or Consulate having jurisdiction over your place of residence for further information. Questions concerning the PRC should be addressed to the office of the USCIS in the applicant’s district of residence in the U.S. If you are in the United Kingdom, you should contact the USCIS office at this Embassy for further information.