"United States" - as Chapter 12 of Title 8 uses that term - to mean the fifty States, the District of Columbia, Puerto Rico, Guam and the U.S. Virgin Islands.
The above is taken from:
DOI Office of Insular Affairs (OIA) - Frequently Asked Questions
Puerto Rico is not a state, but rather a protectorate of the US and people born there are US Citizens. As to the baby according to:
Child Citizenship Act Of 2000
Child Citizenship Act Of 2000
Fact Sheet
Purpose
On February 27, 2001, the Child Citizenship Act of 2000 becomes effective. The aim of this law, which, among other things, amends Section 320 of the Immigration and Nationality Act (INA), is to facilitate the automatic acquisition of U.S. citizenship for both biological and adopted children of U.S. citizens who are born abroad and who do not acquire U.S. citizenship at birth. We are pleased to note that, because of this law, U.S. citizenship will be conferred automatically upon thousands of children currently in the United States.
Requirements
The following are the Act's requirements:
1. At least one parent of the child is a U.S. citizen, either by birth or naturalization.
2. The child is under the age of 18.
3. The child must be residing in the United States in the legal and physical custody of the U.S. citizen parent after having been lawfully admitted into this country as an immigrant for lawful permanent residence.
4. If the child has been adopted, the adoption must be final.
As I read the above Child Citizenship act, there will be some legal work to bring in the child as a US Citizen if it is born in the DR. No such work would be needed if it was born in
PR.