It popped in my head, don't know why.
Let's pretend that your girlfriend/wife is pregnant and could give birth in the very near future. However, you decide to go with her on vacation for 2 weeks in Mexico anyways.
While there, it turns out that the baby needs to come out earlier than planned and she goes to the hospital there to deliver.
Now, here is my question. Is that baby a Mexican citizen now, or still a US Citizen (if the couple originates from the US)
Normally, you are a citizen of the country you are born in.