Answer the question I posed, what interval is allowed to elapse between entering Canada (terminating residence in the US) and giving birth?
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That can be established by one's domicile. (Incidentally, Wong Kim Ark's parents' permanent US domicile was one of the main determinants in the ruling.) Where was the US citizen parent most recently legally living and working? In the US? In a foreign country? In Cruz's mother's case, it was Canada, for 3 years.
Did Cruz's mother maintain a residence in the US and list that as her legal residence on any legal transactions during that time?
This type of thing happens all the time on a smaller scale between states. Some people have residences in more than one state, but only one state is their domicile for legal purposes.