In the majority of situations in which a child is born outside of the United States and one or both of his or her parents are U.S. Citizens, the child will be a U.S. citizen from birth. This article will look at the situations in which children born to U.S. citizen parent(s) abroad are entitled to citizenship from birth, and explain how parents in these situations may go about obtaining proof of their child's citizenship. In any scenarios when the parents are unsure if they will transmit citizenship automatically, they should consult an experienced immigration practitioner if there is any possibility that they may deliver a child abroad.
Please note that this article generally provides rules for when children born abroad to U.S. citizens are U.S. citizens from birth now. For people in some of these scenarios born before November 13, 1986, their claims to citizenship from birth would be evaluated under the statutes that were in force at the time. This article will note some of those cases where it is also applicable to the issue concerning this article, but it is not intended to be a guide for them.
SCENARIOS IN WHICH A CHILD BORN TO U.S. CITIZEN PARENT(S) ABROAD IS A CITIZEN FROM BIRTH
PARENTS ARE MARRIED AND AT LEAST ONE PARENT IS A U.S. CITIZEN
Under current law, a child who is born to married parents and one or both parents are U.S. citizens; the child is a U.S. citizen from birth in the following scenarios:
Pursuant to statute, "physical presence" is determined based on what the laws were for physical presence at the time of the birth. Thus, for any birth occurring overseas now, the above rules on physical presence are the requirements. However, for births occurring between December 24, 1952, and November 13, 1986, the physical presence requirement is 10 years physical presence with at least 5 years occurring after the age of 14. If the parent was naturalized as a U.S. citizen, time before and after naturalization may count in determining physical presence toward 5-year physical presence requirement.
Normally, persons born in American Samoa or Swains Island are U.S. nationals at birth rather than U.S. citizens. However, there is a special provision for children born to a U.S. citizen parent in one of the two U.S. outlying possessions:
PARENTS ARE UNMARRIED AND ONLY MOTHER IS A U.S. CITIZEN
Under current law, children who are born to unmarried parents, with the mother being a U.S. citizen, are U.S. citizens from birth under the following scenario:
PARENTS ARE UNMARRIED AND ONLY FATHER IS A U.S. CITIZEN
The scenario is a bit more complicated if a child is born to unmarried parents and only the father is a U.S. citizen. For all scenarios where a child is born overseas, "presumption of alienage" for persons born outside of the United States that the child's parents must overcome. This can be especially difficult in scenarios where the child's mother is not a U.S. citizen and the child's paternity cannot be easily ascertained. In order for the child to be a U.S. citizen from birth under this scenario, the following must be true:
In cases where the U.S. citizen father is not inclined to cooperate in order to demonstrate that he is the father of a child (such as to establish that the child was entitled to U.S. citizenship from birth), and the non-U.S. citizen mother wants the child to have U.S. citizenship, she should consult with an experienced immigration practitioner who may help her obtain the requisite proof through adjudication that her child was entitled to U.S. citizenship from birth.
SPECIAL CASE: ASSISTED REPRODUCTIVE TECHNOLOGY
In order for a child born using assisted reproductive technology to be a U.S. citizen from birth under the preceding provisions, one of the following must be true:
In most situations, the parents will likely be required to provide details of the circumstances surrounding the child's conception in order to establish that the child was a U.S. citizen from birth.
None of the preceding provisions apply to children adopted by U.S. citizen parents abroad. 
OBTAINING PROOF OF CITIZENSHIP
In situations where a child is born overseas and eligible for citizenship from birth on account of the citizenship of one or both parents, his or her parents should contact the nearest U.S. embassy or consulate to apply for an FS-240, Consular Report of Birth Abroad of a Citizen of the United States of America. The FS-240 serves as proof of citizenship. Since obtaining an FS-240 is the easiest way to obtain proof of citizenship expeditiously, eligible U.S. citizen parents who anticipate that they may give birth to a child overseas are well advised to ensure that they know the location of the nearest U.S. embassy or consulate.
Parents who return to the United States without acquiring an FS-240 may instead file a Form N-600, Application for Certificate of Citizenship at their nearest United States Citizenship and Immigration Services (USCIS) office.
A child who is lawfully a citizen at birth does not need proof of citizenship in order to be a U.S. citizen. However, failure to obtain documentation that a child born to U.S. citizen parents abroad is a U.S. citizen may lead to complications later when seeking to obtain a U.S. passport or registering for a U.S. school.