After a child is born abroad (outside the U.S.) to at least one U.S. Citizen parent.
If you believe that your child (born abroad) has a claim to U.S. Citizenship, it will be necessary for the U.S. citizen parent to appear in person at the Embassy or Consulate in order to execute an application for a “Consular Report of Birth Abroad” before a consular officer. At that time, a passport application may also be executed.
Child, now over age 18, born abroad to at least one U.S. Citizen parent.
If you are the foreign-born child and are now over the age 18, and your birth was not reported to the U.S. Consulate or Embassy, by your U.S. Citizen-parent, you may have a possible claim to U.S. citizenship, if certain requirements are met.
Children born outside the U.S. who may not have a claim to U.S. citizenship – What if I do not meet the requirements for transmission of citizenship to my child?
If you believe that your child does not have a claim to U.S. Citizenship, the nearest Embassy or Consulate in your home country can issue a letter of no-claim to US citizenship.
After receiving the letter of no-claim, it may be possible for your child to apply for Naturalization (acquiring U.S. citizenship by means other than birth), OR an immigrant visa, leading to a Green Card.