Foreign nationals who want to become U.S. immigrants are classified into categories based on a preference system. If you are the parent, spouse or unmarried child under the age of 21 of a U.S. citizen, you do not have to wait for an immigrant visa number to become available once the visa petition filed for you is approved by the United States Citizenship and Immigration Services (USCIS). An immigrant visa number will be immediately available for you. The relatives in the remaining categories must wait for an immigrant visa number to become available according to certain preferences.
You must wait for an immigrant visa number to become available according to the following preferences:
First Preference: Unmarried, adult sons and daughters of U.S. citizens. Adult means 21 years of age or older
Second Preference: Spouse of lawful permanent residents, their unmarried children (under 21), and unmarried sons and daughters of lawful permanent residents
Third Preference: Married sons and daughters of U.S. citizens
Fourth Preference: Brothers and sisters of adult U.S. citizens