Probably a blend, and sometimes both. Keep in mind that there's also a Pregnancy Tourism industry, and thus, "neither" is ALSO a valid answer: The father is some dipshit in Kuwait who knocked up some woman there, and she flew to the USA for a three-week vacation at her estimated birthing window so she could pop one out on the magic dirt and magically gain citizenship.

The traitor there is the customs officials who let clearly ready-to-birth women into a country that has magical dirt.