U.S. citizenship by descent, or acquisition at birth, applies to children born outside the United States to U.S. citizen parents. Requirements vary based on whether one or both parents are citizens, the date of birth, and whether the parents were married. Generally, at least one parent must have been a U.S. citizen at the time of birth and have met certain physical presence requirements in the U.S. before the child's birth. Derivative citizenship may also be acquired through naturalized parents.