The physical presence of a parent in the United States is a very important part of determining whether or not a child is a citizen immediately upon being born.
For example, if the child was born after November 13, 1986, and one parent is a citizen, that child is automatically a citizen. This is only true, though, if that parent has actually been in the territories governed by the U.S. or in the U.S. itself for a minimum of five years. This has to be done before the child is born and two years must have passed after the parent turned 14 years old.
However, parents should know that employment can alter the requirement. For instance, if a person wasn't in the U.S. or the U.S. territories for the full five years, he or she can still count time abroad toward this requirement if the time abroad was spent serving in the military. If the parent spent four years in the United States and then a year serving in Iraq, for example, that would count as five years and the child would be a citizen at birth.
This is also true if the time aboard was spent in the employ of some specific international organizations or of the U.S. government, even in a non-military role.
This illustrates just how important it is to fully understand the requirements for citizenship. Even when they seem very straightforward, there can be exceptions that may alter your child's status. Be very sure that you understand the impact that your employment can have and what it means for your child's future.
Source: U.S. Citizenship and Immigration Services, "Citizenship Through Parents," accessed Nov. 24, 2016