Citizen Parent(s) Citizenship Concerns
A child born abroad to two U.S. citizen parents in wedlock acquires U.S. citizenship at birth, provided that one of the parents resided in the U.S. prior to the child's birth. No specific period of time for such residence is required.
A child born abroad to one U.S. citizen parent and one alien parent in wedlockacquires U.S. citizenship at birth, provided the citizen parent was physically present in the U.S. for the time period required by the law.

Reporting the Birth to Establish Citizenship
The birth of a child abroad to a U.S. citizen parent(s) should be reported as soon as possible to the nearest U.S. embassy or consulate to establish an official record of the child's claim to U.S. citizenship at birth. The official record is a Consular Report of Birth of a Citizen of the United States of America, or Form FS-240. This document, known as the Consular Report of Birth Abroad, is a basic United States citizenship document. An original FS-240 is furnished to the parents at the time the registration is approved. A Consular Report of Birth can be prepared only at a U.S. embassy or consulate. It cannot be prepared if the child has been brought back into the United States (although a different document may be requested – see the next two paragraphs), or, if the child is 18 years of age or older at the time the application is made.

If the child returns to the U.S. without a Form FS-240 being filed, an application may be made for a Certificate of Citizenship. Obtaining this certificate involves presentation of basically the same documentation required to obtain a Consular Report of Birth. Under law, the Consular Report of Birth and the Certificate of Citizenship are equally acceptable as proof of citizenship. File Form N-600 (Application for Certificate of Citizenship) with yournearest USCIS office.

A U.S. passport is also proof of citizenship. 

Legal Permanent Resident Parents
A child born abroad of legal permanent resident parents may enter the U.S. without a visa provided the child is accompanied by a parent upon that parent's initial return to the U.S. within two years of the child's birth with documentation showing the parent-child relationship. If the child is not brought back to the U.S. when the parents initially enter the U.S. (following the birth) within this time period, the child must obtain an immigrant visa to enter the U.S. In this case, the legal permanent resident parent(s) must file an immigrant visa petition with the U.S. Citizenship and Immigration Services.

Requesting a Copy of a Consular Report of Birth
If you were born in a foreign country to a U.S. citizen parent or parents, and your parent registered your birth at a U.S. embassy or consulate in the form of a Consular Report of Birth (Form FS-240), and you would like evidence of your birth and United States citizenship, you should submit a written request which includes the following five items:

Your full name at birth, and date and country of birth.
1. Parents' full names, including mother's maiden name, and their dates and places of birth and nationality.
2. A daytime telephone number.
3. You or your parents' signature. A legal guardian's signature is acceptable only when accompanied by a certified copy of the guardianship papers or court order of adoption.
4. A $40 fee per document. (Check or money order made payable to the Department of State.)

Establishing Citizenship Later
If you were born in a foreign country to a U.S. citizen parent or parents and your parent did not register your birth in the form of a Consular Report of Birth FS-240, you may apply to USCIS for a Certificate of Citizenship, or to a U.S. passport agency for a U.S. passport to document your U.S. citizenship.