The idea of giving birth while aboard an international flight might seem like something out of a Leslie Nielsen movie (except you can call the baby Shirley), but it happens enough to spawn a Quora thread with 37 answers to the question: What’s the kid’s nationality? Sadly, it is not “The Sky.”
The 1961 Convention On The Reduction of Statelessness (awesome read, btw) determined that infants can claim citizenship of wherever the plane was registered, but only if they had no other country of origin. Kids were otherwise covered by jus sanguinis (right of blood) at birth — a principe of nationality law that says citizenship is based on where their parents are from. But some countries like the United States have opted for looser policy of jus soli (right of the soil), which means if your kid is born above that country, they get to keep it. European countries have more restrictive immigration laws, while countries like the U.S., Canada, Brazil, Costa Rica, Guatemala, Panama, and Uruguay observe unrestricted jus soli (read the full list here). Theoretically, if a baby is born to Canadian mother, on an American plane flying over Brazil, the baby can claim citizenship to all of the above. But a infant born to a Canadian mother on a Russian plane over the U.K. is just Canadian, eh.
A 2010 study from the Center For Immigration Studies found that only 30 out of 194 countries including the U.S. offer citizenship automatically based on location, causing some to question if this stance is unusual. However, the research was conducted by an organization that favors more restrictive immigration, and did not have definitive information for 19 countries, so take it with a grain a of salt. The real concern should be what the other passengers would like to drink, since the parents who accidentally brought a screaming newborn onto the plane are clearly buying.